Merge pull request #2288 from prometheus/limit-scrape

Add ability to limit scrape samples, and related metrics

config/config.go

@@ -497,6 +497,8 @@ type ScrapeConfig struct {
 	MetricsPath string `yaml:"metrics_path,omitempty"`
 	// The URL scheme with which to fetch metrics from targets.
 	Scheme string `yaml:"scheme,omitempty"`
+	// More than this many samples post metric-relabelling will cause the scrape to fail.
+	SampleLimit uint `yaml:"sample_limit,omitempty"`
 
 	// We cannot do proper Go type embedding below as the parser will then parse
 	// values arbitrarily into the overflow maps of further-down types.

config/config_test.go

@@ -133,6 +133,7 @@ var expectedConf = &Config{
 
 			ScrapeInterval: model.Duration(50 * time.Second),
 			ScrapeTimeout:  model.Duration(5 * time.Second),
+			SampleLimit:    1000,
 
 			HTTPClientConfig: HTTPClientConfig{
 				BasicAuth: &BasicAuth{

config/testdata/conf.good.yml

@@ -70,6 +70,8 @@ scrape_configs:
   scrape_interval: 50s
   scrape_timeout:  5s
 
+  sample_limit: 1000
+
   metrics_path: /my_path
   scheme: https
 

retrieval/scrape.go

@@ -33,9 +33,10 @@ import (
 )
 
 const (
-	scrapeHealthMetricName   = "up"
-	scrapeDurationMetricName = "scrape_duration_seconds"
-	scrapeSamplesMetricName  = "scrape_samples_scraped"
+	scrapeHealthMetricName       = "up"
+	scrapeDurationMetricName     = "scrape_duration_seconds"
+	scrapeSamplesMetricName      = "scrape_samples_scraped"
+	samplesPostRelabelMetricName = "scrape_samples_post_metric_relabeling"
 )
 
 var (
@@ -76,6 +77,12 @@ var (
 		},
 		[]string{"scrape_job"},
 	)
+	targetScrapeSampleLimit = prometheus.NewCounter(
+		prometheus.CounterOpts{
+			Name: "prometheus_target_scrapes_exceeded_sample_limit_total",
+			Help: "Total number of scrapes that hit the sample limit and were rejected.",
+		},
+	)
 )
 
 func init() {
@@ -84,6 +91,7 @@ func init() {
 	prometheus.MustRegister(targetReloadIntervalLength)
 	prometheus.MustRegister(targetSyncIntervalLength)
 	prometheus.MustRegister(targetScrapePoolSyncsCounter)
+	prometheus.MustRegister(targetScrapeSampleLimit)
 }
 
 // scrapePool manages scrapes for sets of targets.
@@ -101,7 +109,7 @@ type scrapePool struct {
 	loops   map[uint64]loop
 
 	// Constructor for new scrape loops. This is settable for testing convenience.
-	newLoop func(context.Context, scraper, storage.SampleAppender, storage.SampleAppender) loop
+	newLoop func(context.Context, scraper, storage.SampleAppender, model.LabelSet, *config.ScrapeConfig) loop
 }
 
 func newScrapePool(ctx context.Context, cfg *config.ScrapeConfig, app storage.SampleAppender) *scrapePool {
@@ -170,7 +178,7 @@ func (sp *scrapePool) reload(cfg *config.ScrapeConfig) {
 		var (
 			t       = sp.targets[fp]
 			s       = &targetScraper{Target: t, client: sp.client}
-			newLoop = sp.newLoop(sp.ctx, s, sp.sampleAppender(t), sp.reportAppender(t))
+			newLoop = sp.newLoop(sp.ctx, s, sp.appender, t.Labels(), sp.config)
 		)
 		wg.Add(1)
 
@@ -231,7 +239,7 @@ func (sp *scrapePool) sync(targets []*Target) {
 
 		if _, ok := sp.targets[hash]; !ok {
 			s := &targetScraper{Target: t, client: sp.client}
-			l := sp.newLoop(sp.ctx, s, sp.sampleAppender(t), sp.reportAppender(t))
+			l := sp.newLoop(sp.ctx, s, sp.appender, t.Labels(), sp.config)
 
 			sp.targets[hash] = t
 			sp.loops[hash] = l
@@ -263,40 +271,6 @@ func (sp *scrapePool) sync(targets []*Target) {
 	wg.Wait()
 }
 
-// sampleAppender returns an appender for ingested samples from the target.
-func (sp *scrapePool) sampleAppender(target *Target) storage.SampleAppender {
-	app := sp.appender
-	// The relabelAppender has to be inside the label-modifying appenders
-	// so the relabeling rules are applied to the correct label set.
-	if mrc := sp.config.MetricRelabelConfigs; len(mrc) > 0 {
-		app = relabelAppender{
-			SampleAppender: app,
-			relabelings:    mrc,
-		}
-	}
-
-	if sp.config.HonorLabels {
-		app = honorLabelsAppender{
-			SampleAppender: app,
-			labels:         target.Labels(),
-		}
-	} else {
-		app = ruleLabelsAppender{
-			SampleAppender: app,
-			labels:         target.Labels(),
-		}
-	}
-	return app
-}
-
-// reportAppender returns an appender for reporting samples for the target.
-func (sp *scrapePool) reportAppender(target *Target) storage.SampleAppender {
-	return ruleLabelsAppender{
-		SampleAppender: sp.appender,
-		labels:         target.Labels(),
-	}
-}
-
 // A scraper retrieves samples and accepts a status report at the end.
 type scraper interface {
 	scrape(ctx context.Context, ts time.Time) (model.Samples, error)
@@ -364,20 +338,34 @@ type loop interface {
 type scrapeLoop struct {
 	scraper scraper
 
-	appender       storage.SampleAppender
-	reportAppender storage.SampleAppender
+	// Where samples are ultimately sent.
+	appender storage.SampleAppender
+
+	targetLabels         model.LabelSet
+	metricRelabelConfigs []*config.RelabelConfig
+	honorLabels          bool
+	sampleLimit          uint
 
 	done   chan struct{}
 	ctx    context.Context
 	cancel func()
 }
 
-func newScrapeLoop(ctx context.Context, sc scraper, app, reportApp storage.SampleAppender) loop {
+func newScrapeLoop(
+	ctx context.Context,
+	sc scraper,
+	appender storage.SampleAppender,
+	targetLabels model.LabelSet,
+	config *config.ScrapeConfig,
+) loop {
 	sl := &scrapeLoop{
-		scraper:        sc,
-		appender:       app,
-		reportAppender: reportApp,
-		done:           make(chan struct{}),
+		scraper:              sc,
+		appender:             appender,
+		targetLabels:         targetLabels,
+		metricRelabelConfigs: config.MetricRelabelConfigs,
+		honorLabels:          config.HonorLabels,
+		sampleLimit:          config.SampleLimit,
+		done:                 make(chan struct{}),
 	}
 	sl.ctx, sl.cancel = context.WithCancel(ctx)
 
@@ -408,8 +396,9 @@ func (sl *scrapeLoop) run(interval, timeout time.Duration, errc chan<- error) {
 
 		if !sl.appender.NeedsThrottling() {
 			var (
-				start        = time.Now()
-				scrapeCtx, _ = context.WithTimeout(sl.ctx, timeout)
+				start                 = time.Now()
+				scrapeCtx, _          = context.WithTimeout(sl.ctx, timeout)
+				numPostRelabelSamples = 0
 			)
 
 			// Only record after the first scrape.
@@ -421,12 +410,12 @@ func (sl *scrapeLoop) run(interval, timeout time.Duration, errc chan<- error) {
 
 			samples, err := sl.scraper.scrape(scrapeCtx, start)
 			if err == nil {
-				sl.append(samples)
-			} else if errc != nil {
+				numPostRelabelSamples, err = sl.append(samples)
+			}
+			if err != nil && errc != nil {
 				errc <- err
 			}
-
-			sl.report(start, time.Since(start), len(samples), err)
+			sl.report(start, time.Since(start), len(samples), numPostRelabelSamples, err)
 			last = start
 		} else {
 			targetSkippedScrapes.Inc()
@@ -445,14 +434,73 @@ func (sl *scrapeLoop) stop() {
 	<-sl.done
 }
 
-func (sl *scrapeLoop) append(samples model.Samples) {
+// wrapAppender wraps a SampleAppender for relabeling. It returns the wrappend
+// appender and an innermost countingAppender that counts the samples actually
+// appended in the end.
+func (sl *scrapeLoop) wrapAppender(app storage.SampleAppender) (storage.SampleAppender, *countingAppender) {
+	// Innermost appender is a countingAppender to count how many samples
+	// are left in the end.
+	countingAppender := &countingAppender{
+		SampleAppender: app,
+	}
+	app = countingAppender
+
+	// The relabelAppender has to be inside the label-modifying appenders so
+	// the relabeling rules are applied to the correct label set.
+	if len(sl.metricRelabelConfigs) > 0 {
+		app = relabelAppender{
+			SampleAppender: app,
+			relabelings:    sl.metricRelabelConfigs,
+		}
+	}
+
+	if sl.honorLabels {
+		app = honorLabelsAppender{
+			SampleAppender: app,
+			labels:         sl.targetLabels,
+		}
+	} else {
+		app = ruleLabelsAppender{
+			SampleAppender: app,
+			labels:         sl.targetLabels,
+		}
+	}
+	return app, countingAppender
+}
+
+func (sl *scrapeLoop) append(samples model.Samples) (int, error) {
 	var (
 		numOutOfOrder = 0
 		numDuplicates = 0
+		app           = sl.appender
+		countingApp   *countingAppender
 	)
 
+	if sl.sampleLimit > 0 {
+		// We need to check for the sample limit, so append everything
+		// to a wrapped bufferAppender first. Then point samples to the
+		// result.
+		bufApp := &bufferAppender{buffer: make(model.Samples, 0, len(samples))}
+		var wrappedBufApp storage.SampleAppender
+		wrappedBufApp, countingApp = sl.wrapAppender(bufApp)
+		for _, s := range samples {
+			// Ignore errors as bufferedAppender always succeds.
+			wrappedBufApp.Append(s)
+		}
+		samples = bufApp.buffer
+		if uint(countingApp.count) > sl.sampleLimit {
+			targetScrapeSampleLimit.Inc()
+			return countingApp.count, fmt.Errorf(
+				"%d samples exceeded limit of %d", countingApp.count, sl.sampleLimit,
+			)
+		}
+	} else {
+		// No need to check for sample limit. Wrap sl.appender directly.
+		app, countingApp = sl.wrapAppender(sl.appender)
+	}
+
 	for _, s := range samples {
-		if err := sl.appender.Append(s); err != nil {
+		if err := app.Append(s); err != nil {
 			switch err {
 			case local.ErrOutOfOrderSample:
 				numOutOfOrder++
@@ -471,9 +519,10 @@ func (sl *scrapeLoop) append(samples model.Samples) {
 	if numDuplicates > 0 {
 		log.With("numDropped", numDuplicates).Warn("Error on ingesting samples with different value but same timestamp")
 	}
+	return countingApp.count, nil
 }
 
-func (sl *scrapeLoop) report(start time.Time, duration time.Duration, scrapedSamples int, err error) {
+func (sl *scrapeLoop) report(start time.Time, duration time.Duration, scrapedSamples, postRelabelSamples int, err error) {
 	sl.scraper.report(start, duration, err)
 
 	ts := model.TimeFromUnixNano(start.UnixNano())
@@ -504,14 +553,29 @@ func (sl *scrapeLoop) report(start time.Time, duration time.Duration, scrapedSam
 		Timestamp: ts,
 		Value:     model.SampleValue(scrapedSamples),
 	}
+	postRelabelSample := &model.Sample{
+		Metric: model.Metric{
+			model.MetricNameLabel: samplesPostRelabelMetricName,
+		},
+		Timestamp: ts,
+		Value:     model.SampleValue(postRelabelSamples),
+	}
 
-	if err := sl.reportAppender.Append(healthSample); err != nil {
+	reportAppender := ruleLabelsAppender{
+		SampleAppender: sl.appender,
+		labels:         sl.targetLabels,
+	}
+
+	if err := reportAppender.Append(healthSample); err != nil {
 		log.With("sample", healthSample).With("error", err).Warn("Scrape health sample discarded")
 	}
-	if err := sl.reportAppender.Append(durationSample); err != nil {
+	if err := reportAppender.Append(durationSample); err != nil {
 		log.With("sample", durationSample).With("error", err).Warn("Scrape duration sample discarded")
 	}
-	if err := sl.reportAppender.Append(countSample); err != nil {
+	if err := reportAppender.Append(countSample); err != nil {
 		log.With("sample", durationSample).With("error", err).Warn("Scrape sample count sample discarded")
 	}
+	if err := reportAppender.Append(postRelabelSample); err != nil {
+		log.With("sample", durationSample).With("error", err).Warn("Scrape sample count post-relabeling sample discarded")
+	}
 }

retrieval/scrape_test.go

@@ -139,7 +139,7 @@ func TestScrapePoolReload(t *testing.T) {
 	}
 	// On starting to run, new loops created on reload check whether their preceding
 	// equivalents have been stopped.
-	newLoop := func(ctx context.Context, s scraper, app, reportApp storage.SampleAppender) loop {
+	newLoop := func(ctx context.Context, s scraper, app storage.SampleAppender, tl model.LabelSet, cfg *config.ScrapeConfig) loop {
 		l := &testLoop{}
 		l.startFunc = func(interval, timeout time.Duration, errc chan<- error) {
 			if interval != 3*time.Second {
@@ -222,44 +222,19 @@ func TestScrapePoolReload(t *testing.T) {
 	}
 }
 
-func TestScrapePoolReportAppender(t *testing.T) {
+func TestScrapeLoopWrapSampleAppender(t *testing.T) {
 	cfg := &config.ScrapeConfig{
 		MetricRelabelConfigs: []*config.RelabelConfig{
-			{}, {}, {},
-		},
-	}
-	target := newTestTarget("example.com:80", 10*time.Millisecond, nil)
-	app := &nopAppender{}
-
-	sp := newScrapePool(context.Background(), cfg, app)
-
-	cfg.HonorLabels = false
-	wrapped := sp.reportAppender(target)
-
-	rl, ok := wrapped.(ruleLabelsAppender)
-	if !ok {
-		t.Fatalf("Expected ruleLabelsAppender but got %T", wrapped)
-	}
-	if rl.SampleAppender != app {
-		t.Fatalf("Expected base appender but got %T", rl.SampleAppender)
-	}
-
-	cfg.HonorLabels = true
-	wrapped = sp.reportAppender(target)
-
-	hl, ok := wrapped.(ruleLabelsAppender)
-	if !ok {
-		t.Fatalf("Expected ruleLabelsAppender but got %T", wrapped)
-	}
-	if hl.SampleAppender != app {
-		t.Fatalf("Expected base appender but got %T", hl.SampleAppender)
-	}
-}
-
-func TestScrapePoolSampleAppender(t *testing.T) {
-	cfg := &config.ScrapeConfig{
-		MetricRelabelConfigs: []*config.RelabelConfig{
-			{}, {}, {},
+			{
+				Action:       config.RelabelDrop,
+				SourceLabels: model.LabelNames{"__name__"},
+				Regex:        config.MustNewRegexp("does_not_match_.*"),
+			},
+			{
+				Action:       config.RelabelDrop,
+				SourceLabels: model.LabelNames{"__name__"},
+				Regex:        config.MustNewRegexp("does_not_match_either_*"),
+			},
 		},
 	}
 
@@ -269,7 +244,15 @@ func TestScrapePoolSampleAppender(t *testing.T) {
 	sp := newScrapePool(context.Background(), cfg, app)
 
 	cfg.HonorLabels = false
-	wrapped := sp.sampleAppender(target)
+
+	sl := sp.newLoop(
+		sp.ctx,
+		&targetScraper{Target: target, client: sp.client},
+		sp.appender,
+		target.Labels(),
+		sp.config,
+	).(*scrapeLoop)
+	wrapped, _ := sl.wrapAppender(sl.appender)
 
 	rl, ok := wrapped.(ruleLabelsAppender)
 	if !ok {
@@ -279,12 +262,23 @@ func TestScrapePoolSampleAppender(t *testing.T) {
 	if !ok {
 		t.Fatalf("Expected relabelAppender but got %T", rl.SampleAppender)
 	}
-	if re.SampleAppender != app {
-		t.Fatalf("Expected base appender but got %T", re.SampleAppender)
+	co, ok := re.SampleAppender.(*countingAppender)
+	if !ok {
+		t.Fatalf("Expected *countingAppender but got %T", re.SampleAppender)
+	}
+	if co.SampleAppender != app {
+		t.Fatalf("Expected base appender but got %T", co.SampleAppender)
 	}
 
 	cfg.HonorLabels = true
-	wrapped = sp.sampleAppender(target)
+	sl = sp.newLoop(
+		sp.ctx,
+		&targetScraper{Target: target, client: sp.client},
+		sp.appender,
+		target.Labels(),
+		sp.config,
+	).(*scrapeLoop)
+	wrapped, _ = sl.wrapAppender(sl.appender)
 
 	hl, ok := wrapped.(honorLabelsAppender)
 	if !ok {
@@ -294,17 +288,176 @@ func TestScrapePoolSampleAppender(t *testing.T) {
 	if !ok {
 		t.Fatalf("Expected relabelAppender but got %T", hl.SampleAppender)
 	}
-	if re.SampleAppender != app {
-		t.Fatalf("Expected base appender but got %T", re.SampleAppender)
+	co, ok = re.SampleAppender.(*countingAppender)
+	if !ok {
+		t.Fatalf("Expected *countingAppender but got %T", re.SampleAppender)
 	}
+	if co.SampleAppender != app {
+		t.Fatalf("Expected base appender but got %T", co.SampleAppender)
+	}
+}
+
+func TestScrapeLoopSampleProcessing(t *testing.T) {
+	readSamples := model.Samples{
+		{
+			Metric: model.Metric{"__name__": "a_metric"},
+		},
+		{
+			Metric: model.Metric{"__name__": "b_metric"},
+		},
+	}
+
+	testCases := []struct {
+		scrapedSamples                  model.Samples
+		scrapeConfig                    *config.ScrapeConfig
+		expectedReportedSamples         model.Samples
+		expectedPostRelabelSamplesCount int
+	}{
+		{ // 0
+			scrapedSamples: readSamples,
+			scrapeConfig:   &config.ScrapeConfig{},
+			expectedReportedSamples: model.Samples{
+				{
+					Metric: model.Metric{"__name__": "up"},
+					Value:  1,
+				},
+				{
+					Metric: model.Metric{"__name__": "scrape_duration_seconds"},
+					Value:  42,
+				},
+				{
+					Metric: model.Metric{"__name__": "scrape_samples_scraped"},
+					Value:  2,
+				},
+				{
+					Metric: model.Metric{"__name__": "scrape_samples_post_metric_relabeling"},
+					Value:  2,
+				},
+			},
+			expectedPostRelabelSamplesCount: 2,
+		},
+		{ // 1
+			scrapedSamples: readSamples,
+			scrapeConfig: &config.ScrapeConfig{
+				MetricRelabelConfigs: []*config.RelabelConfig{
+					{
+						Action:       config.RelabelDrop,
+						SourceLabels: model.LabelNames{"__name__"},
+						Regex:        config.MustNewRegexp("a.*"),
+					},
+				},
+			},
+			expectedReportedSamples: model.Samples{
+				{
+					Metric: model.Metric{"__name__": "up"},
+					Value:  1,
+				},
+				{
+					Metric: model.Metric{"__name__": "scrape_duration_seconds"},
+					Value:  42,
+				},
+				{
+					Metric: model.Metric{"__name__": "scrape_samples_scraped"},
+					Value:  2,
+				},
+				{
+					Metric: model.Metric{"__name__": "scrape_samples_post_metric_relabeling"},
+					Value:  1,
+				},
+			},
+			expectedPostRelabelSamplesCount: 1,
+		},
+		{ // 2
+			scrapedSamples: readSamples,
+			scrapeConfig: &config.ScrapeConfig{
+				SampleLimit: 1,
+				MetricRelabelConfigs: []*config.RelabelConfig{
+					{
+						Action:       config.RelabelDrop,
+						SourceLabels: model.LabelNames{"__name__"},
+						Regex:        config.MustNewRegexp("a.*"),
+					},
+				},
+			},
+			expectedReportedSamples: model.Samples{
+				{
+					Metric: model.Metric{"__name__": "up"},
+					Value:  1,
+				},
+				{
+					Metric: model.Metric{"__name__": "scrape_duration_seconds"},
+					Value:  42,
+				},
+				{
+					Metric: model.Metric{"__name__": "scrape_samples_scraped"},
+					Value:  2,
+				},
+				{
+					Metric: model.Metric{"__name__": "scrape_samples_post_metric_relabeling"},
+					Value:  1,
+				},
+			},
+			expectedPostRelabelSamplesCount: 1,
+		},
+		{ // 3
+			scrapedSamples: readSamples,
+			scrapeConfig: &config.ScrapeConfig{
+				SampleLimit: 1,
+			},
+			expectedReportedSamples: model.Samples{
+				{
+					Metric: model.Metric{"__name__": "up"},
+					Value:  0,
+				},
+				{
+					Metric: model.Metric{"__name__": "scrape_duration_seconds"},
+					Value:  42,
+				},
+				{
+					Metric: model.Metric{"__name__": "scrape_samples_scraped"},
+					Value:  2,
+				},
+				{
+					Metric: model.Metric{"__name__": "scrape_samples_post_metric_relabeling"},
+					Value:  2,
+				},
+			},
+			expectedPostRelabelSamplesCount: 2,
+		},
+	}
+
+	for i, test := range testCases {
+		ingestedSamples := &bufferAppender{buffer: model.Samples{}}
+
+		target := newTestTarget("example.com:80", 10*time.Millisecond, nil)
+
+		scraper := &testScraper{}
+		sl := newScrapeLoop(context.Background(), scraper, ingestedSamples, target.Labels(), test.scrapeConfig).(*scrapeLoop)
+		num, err := sl.append(test.scrapedSamples)
+		sl.report(time.Unix(0, 0), 42*time.Second, len(test.scrapedSamples), num, err)
+		reportedSamples := ingestedSamples.buffer
+		if err == nil {
+			reportedSamples = reportedSamples[num:]
+		}
+
+		if !reflect.DeepEqual(reportedSamples, test.expectedReportedSamples) {
+			t.Errorf("Reported samples did not match expected metrics for case %d", i)
+			t.Errorf("Expected: %v", test.expectedReportedSamples)
+			t.Fatalf("Got: %v", reportedSamples)
+		}
+		if test.expectedPostRelabelSamplesCount != num {
+			t.Fatalf("Case %d: Ingested samples %d did not match expected value %d", i, num, test.expectedPostRelabelSamplesCount)
+		}
+	}
+
 }
 
 func TestScrapeLoopStop(t *testing.T) {
 	scraper := &testScraper{}
-	sl := newScrapeLoop(context.Background(), scraper, nil, nil)
+	sl := newScrapeLoop(context.Background(), scraper, nil, nil, &config.ScrapeConfig{})
 
 	// The scrape pool synchronizes on stopping scrape loops. However, new scrape
-	// loops are syarted asynchronously. Thus it's possible, that a loop is stopped
+	// loops are started asynchronously. Thus it's possible, that a loop is stopped
 	// again before having started properly.
 	// Stopping not-yet-started loops must block until the run method was called and exited.
 	// The run method must exit immediately.
@@ -351,14 +504,13 @@ func TestScrapeLoopRun(t *testing.T) {
 		signal = make(chan struct{})
 		errc   = make(chan error)
 
-		scraper   = &testScraper{}
-		app       = &nopAppender{}
-		reportApp = &nopAppender{}
+		scraper = &testScraper{}
+		app     = &nopAppender{}
 	)
 	defer close(signal)
 
 	ctx, cancel := context.WithCancel(context.Background())
-	sl := newScrapeLoop(ctx, scraper, app, reportApp)
+	sl := newScrapeLoop(ctx, scraper, app, nil, &config.ScrapeConfig{})
 
 	// The loop must terminate during the initial offset if the context
 	// is canceled.
@@ -396,7 +548,7 @@ func TestScrapeLoopRun(t *testing.T) {
 	}
 
 	ctx, cancel = context.WithCancel(context.Background())
-	sl = newScrapeLoop(ctx, scraper, app, reportApp)
+	sl = newScrapeLoop(ctx, scraper, app, nil, &config.ScrapeConfig{})
 
 	go func() {
 		sl.run(time.Second, 100*time.Millisecond, errc)

retrieval/target.go

@@ -278,6 +278,29 @@ func (app relabelAppender) Append(s *model.Sample) error {
 	return app.SampleAppender.Append(s)
 }
 
+// bufferAppender appends samples to the given buffer.
+type bufferAppender struct {
+	buffer model.Samples
+}
+
+func (app *bufferAppender) Append(s *model.Sample) error {
+	app.buffer = append(app.buffer, s)
+	return nil
+}
+
+func (app *bufferAppender) NeedsThrottling() bool { return false }
+
+// countingAppender counts the samples appended to the underlying appender.
+type countingAppender struct {
+	storage.SampleAppender
+	count int
+}
+
+func (app *countingAppender) Append(s *model.Sample) error {
+	app.count++
+	return app.SampleAppender.Append(s)
+}
+
 // populateLabels builds a label set from the given label set and scrape configuration.
 // It returns a label set before relabeling was applied as the second return value.
 // Returns a nil label set if the target is dropped during relabeling.