Added DNS-SD lookup counter for successful/unsuccessful lookups

Change-Id: I0a71e994a989cecace280b5134a31ebc2ace7591

retrieval/instrumentation.go

@@ -42,10 +42,20 @@ var (
 		ReportablePercentiles: []float64{0.01, 0.05, 0.5, 0.90, 0.99}})
 
 	targetOperations = prometheus.NewCounter()
+	dnsSDLookupsCount = prometheus.NewCounter()
 )
 
+func recordOutcome(err error) {
+	message := success
+	if err != nil {
+		message = failure
+	}
+	dnsSDLookupsCount.Increment(map[string]string{outcome: message})
+}
+
 func init() {
 	prometheus.Register("prometheus_target_operations_total", "The total numbers of operations of the various targets that are being monitored.", prometheus.NilLabels, targetOperations)
 	prometheus.Register("prometheus_target_operation_latency_ms", "The latencies for various target operations.", prometheus.NilLabels, targetOperationLatencies)
 	prometheus.Register("prometheus_targetpool_duration_ms", "The durations for each TargetPool to retrieve state from all included entities.", prometheus.NilLabels, retrievalDurations)
+	prometheus.Register("prometheus_dns_sd_lookups_total", "The number of DNS-SD lookup successes/failures per pool.", prometheus.NilLabels, dnsSDLookupsCount)
 }

retrieval/target_provider.go

@@ -59,11 +59,15 @@ func NewSdTargetProvider(job config.JobConfig) *sdTargetProvider {
 }
 
 func (p *sdTargetProvider) Targets() ([]Target, error) {
+	var err error
+	defer func() { recordOutcome(err) }()
+
 	if time.Since(p.lastRefresh) < p.refreshInterval {
 		return p.targets, nil
 	}
 
 	response, err := lookupSRV(p.job.GetSdName())
+
 	if err != nil {
 		return nil, err
 	}