add status code to request duration metric

middlewares/metrics.go

@@ -42,9 +42,12 @@ func (m *MetricsWrapper) ServeHTTP(rw http.ResponseWriter, r *http.Request, next
 	start := time.Now()
 	prw := &responseRecorder{rw, http.StatusOK}
 	next(prw, r)
-	labels := []string{"code", strconv.Itoa(prw.statusCode), "method", r.Method}
-	m.Impl.getReqsCounter().With(labels...).Add(1)
-	m.Impl.getReqDurationHistogram().Observe(float64(time.Since(start).Seconds()))
+
+	reqLabels := []string{"code", strconv.Itoa(prw.statusCode), "method", r.Method}
+	m.Impl.getReqsCounter().With(reqLabels...).Add(1)
+
+	reqDurationLabels := []string{"code", strconv.Itoa(prw.statusCode)}
+	m.Impl.getReqDurationHistogram().With(reqDurationLabels...).Observe(float64(time.Since(start).Seconds()))
 }
 
 // MetricsRetryListener is an implementation of the RetryListener interface to

middlewares/prometheus.go

@@ -17,7 +17,7 @@ const (
 
 // Prometheus is an Implementation for Metrics that exposes the following Prometheus metrics:
 // - number of requests partitioned by status code and method
-// - request durations
+// - request durations partitioned by status code
 // - amount of retries happened
 type Prometheus struct {
 	reqsCounter          metrics.Counter
@@ -73,7 +73,7 @@ func NewPrometheus(name string, config *types.Prometheus) (*Prometheus, []stdpro
 			ConstLabels: stdprometheus.Labels{"service": name},
 			Buckets:     buckets,
 		},
-		[]string{},
+		[]string{"code"},
 	)
 	hv, err = registerHistogramVec(hv)
 	if err != nil {

middlewares/prometheus_test.go

@@ -75,6 +75,7 @@ func TestPrometheus(t *testing.T) {
 			name: reqDurationName,
 			labels: map[string]string{
 				"service": "test",
+				"code":    "200",
 			},
 			assert: func(family *dto.MetricFamily) {
 				sc := family.Metric[0].Histogram.GetSampleCount()