Change execution_duration_seconds metric type from gauge to histogram (…

…#169)

* change duration seconds to histogram; fix registerer wrapper; fix typo; fix time.Now() to time.Time{}

* add comment for return time.Time{}

metrics/metrics.go

@@ -24,7 +24,7 @@ type PerformanceMetric interface {
 
 type performanceMetric struct {
 	executionStarted         *prometheus.GaugeVec
-	executionDurationSeconds *prometheus.GaugeVec
+	executionDurationSeconds *prometheus.HistogramVec
 	executionSucceededTotal  *prometheus.CounterVec
 	executionFailedTotal     *prometheus.CounterVec
 }
@@ -36,22 +36,22 @@ func NewPerformanceMetric(namespace string, labels prometheus.Labels, reg promet
 		Help:      "Last Unix time when execution started.",
 	}, nil)
 
-	executionDurationSeconds := prometheus.NewGaugeVec(prometheus.GaugeOpts{
+	executionDurationSeconds := prometheus.NewHistogramVec(prometheus.HistogramOpts{
 		Namespace: namespace,
 		Name:      executionDurationSecondsKey,
-		Help:      "Duration of the last execution.",
+		Help:      "Duration of the executions.",
 	}, nil)
 
 	executionSucceededTotal := prometheus.NewCounterVec(prometheus.CounterOpts{
 		Namespace: namespace,
 		Name:      executionSucceededTotalKey,
-		Help:      "Total number of the executions wich succeeded.",
+		Help:      "Total number of the executions which succeeded.",
 	}, nil)
 
 	executionFailedTotal := prometheus.NewCounterVec(prometheus.CounterOpts{
 		Namespace: namespace,
 		Name:      executionFailedTotalKey,
-		Help:      "Total number of the executions wich failed.",
+		Help:      "Total number of the executions which failed.",
 	}, nil)
 
 	Register(labels, reg, executionStarted, executionDurationSeconds, executionSucceededTotal, executionFailedTotal)
@@ -72,7 +72,7 @@ func (m *performanceMetric) Start() time.Time {
 
 func (m *performanceMetric) Duration(start time.Time) {
 	duration := time.Since(start)
-	m.executionDurationSeconds.WithLabelValues().Set(duration.Seconds())
+	m.executionDurationSeconds.WithLabelValues().Observe(duration.Seconds())
 }
 
 func (m *performanceMetric) Success() {
@@ -88,8 +88,9 @@ func (m *performanceMetric) Failure() {
 type NullablePerformanceMetric struct{}
 
 func (NullablePerformanceMetric) Start() time.Time {
-	return time.Now()
+	// NullablePerformanceMetric is a no-op, so returning empty value
+	return time.Time{}
 }
-func (NullablePerformanceMetric) Duration(start time.Time) {}
-func (NullablePerformanceMetric) Success()                 {}
-func (NullablePerformanceMetric) Failure()                 {}
+func (NullablePerformanceMetric) Duration(_ time.Time) {}
+func (NullablePerformanceMetric) Success()             {}
+func (NullablePerformanceMetric) Failure()             {}

metrics/register.go

@@ -2,12 +2,14 @@ package metrics
 
 import (
 	"github.com/prometheus/client_golang/prometheus"
+
 	"github.com/trustwallet/go-libs/logging"
 )
 
 func Register(labels prometheus.Labels, reg prometheus.Registerer, collectors ...prometheus.Collector) {
+	registerer := prometheus.WrapRegistererWith(labels, reg)
 	for _, c := range collectors {
-		err := prometheus.WrapRegistererWith(labels, reg).Register(c)
+		err := registerer.Register(c)
 		if err != nil {
 			if _, ok := err.(*prometheus.AlreadyRegisteredError); !ok {
 				logging.GetLogger().WithError(err).