Add request_duration_seconds metric

metrics.go

@@ -17,7 +17,7 @@ const metricNamespace = "rushlink"
 // metricURLsTotalGauge counts the number of requests that are handled by
 // the application, partitioned by status code and HTTP method.
 //
-// This counter is updated by throughout the application.
+// This counter is updated by the router.
 var metricRequestsTotalCounter = prometheus.NewCounterVec(
 	prometheus.CounterOpts{
 		Namespace: metricNamespace,
@@ -26,6 +26,27 @@ var metricRequestsTotalCounter = prometheus.NewCounterVec(
 		Help:      "How many HTTP requests processed, partitioned by status code and HTTP method.",
 	}, []string{"code", "method"})
 
+// metricRequestsLatencyNanoSeconds keeps track of the request latencies for
+// each http request.
+//
+// This historogram is updated by the router.
+var metricRequestsLatencyNanoSeconds = prometheus.NewHistogramVec(
+	prometheus.HistogramOpts{
+		Namespace: metricNamespace,
+		Subsystem: "http",
+		Name:      "request_duration_seconds",
+		Buckets: []float64{
+			float64(500e-6),
+			float64(1e-3),
+			float64(2e-3),
+			float64(5e-3),
+			float64(10e-3),
+			float64(20e-3),
+			float64(50e-3),
+		},
+		Help: "The latency of each HTTP request, partitioned by status code and HTTP method.",
+	}, []string{"code", "method"})
+
 // metricURLsTotalGauge measures the amount of pastes stored in the database,
 // partitioned by type and state.
 //
@@ -41,6 +62,7 @@ var metricURLsTotalGauge = prometheus.NewGaugeVec(
 // StartMetricsServer starts sering Prometheus metrics exports on addr
 func StartMetricsServer(addr string, database *db.Database, fs *db.FileStore) {
 	prometheus.MustRegister(metricRequestsTotalCounter)
+	prometheus.MustRegister(metricRequestsLatencyNanoSeconds)
 	prometheus.MustRegister(metricURLsTotalGauge)
 
 	router := mux.NewRouter()