[BUGFIX] Scraping: allow multiple samples on same series (#14685)

So long as they specify timestamps. We don't check that the timestamps
are different.

Extend test, and use client_golang/prometheus/testutil to simplify metric check.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

scrape/scrape.go

@@ -1631,7 +1631,7 @@ loop:
 			updateMetadata(lset, true)
 		}
 
-		if seriesAlreadyScraped {
+		if seriesAlreadyScraped && parsedTimestamp == nil {
 			err = storage.ErrDuplicateSampleForTimestamp
 		} else {
 			if ctMs := p.CreatedTimestamp(); sl.enableCTZeroIngestion && ctMs != nil {

scrape/scrape_test.go

@@ -35,6 +35,7 @@ import (
 	"github.com/gogo/protobuf/proto"
 	"github.com/google/go-cmp/cmp"
 	"github.com/prometheus/client_golang/prometheus"
+	prom_testutil "github.com/prometheus/client_golang/prometheus/testutil"
 	dto "github.com/prometheus/client_model/go"
 	config_util "github.com/prometheus/common/config"
 	"github.com/prometheus/common/model"
@@ -3627,6 +3628,7 @@ func TestScrapeLoopSeriesAddedDuplicates(t *testing.T) {
 	require.Equal(t, 3, total)
 	require.Equal(t, 3, added)
 	require.Equal(t, 1, seriesAdded)
+	require.Equal(t, 2.0, prom_testutil.ToFloat64(sl.metrics.targetScrapeSampleDuplicate))
 
 	slApp = sl.appender(ctx)
 	total, added, seriesAdded, err = sl.append(slApp, []byte("test_metric 1\ntest_metric 1\ntest_metric 1\n"), "", time.Time{})
@@ -3635,12 +3637,18 @@ func TestScrapeLoopSeriesAddedDuplicates(t *testing.T) {
 	require.Equal(t, 3, total)
 	require.Equal(t, 3, added)
 	require.Equal(t, 0, seriesAdded)
+	require.Equal(t, 4.0, prom_testutil.ToFloat64(sl.metrics.targetScrapeSampleDuplicate))
 
-	metric := dto.Metric{}
-	err = sl.metrics.targetScrapeSampleDuplicate.Write(&metric)
+	// When different timestamps are supplied, multiple samples are accepted.
+	slApp = sl.appender(ctx)
+	total, added, seriesAdded, err = sl.append(slApp, []byte("test_metric 1 1001\ntest_metric 1 1002\ntest_metric 1 1003\n"), "", time.Time{})
 	require.NoError(t, err)
-	value := metric.GetCounter().GetValue()
-	require.Equal(t, 4.0, value)
+	require.NoError(t, slApp.Commit())
+	require.Equal(t, 3, total)
+	require.Equal(t, 3, added)
+	require.Equal(t, 0, seriesAdded)
+	// Metric is not higher than last time.
+	require.Equal(t, 4.0, prom_testutil.ToFloat64(sl.metrics.targetScrapeSampleDuplicate))
 }
 
 // This tests running a full scrape loop and checking that the scrape option