From c4ff0ab1b7501a66732cd923f999032f4f3ffe32 Mon Sep 17 00:00:00 2001
From: Daan Sprenkels <daan@dsprenkels.com>
Date: Sun, 16 May 2021 20:24:00 +0200
Subject: [PATCH] Add request_duration_seconds metric

---
 metrics.go | 24 +++++++++++++++++++++++-
 router.go  |  5 +++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/metrics.go b/metrics.go
index 0df6acf..d3b629e 100644
--- a/metrics.go
+++ b/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()
diff --git a/router.go b/router.go
index d9e5df4..6e888a3 100644
--- a/router.go
+++ b/router.go
@@ -58,13 +58,18 @@ func (rl *rushlink) recoveryMiddleware(next http.Handler) http.Handler {
 
 func (rl *rushlink) metricsMiddleware(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		tick := time.Now()
 		srw := statusResponseWriter{Inner: w}
 		next.ServeHTTP(&srw, r)
+		tock := time.Now()
 
 		status := strconv.Itoa(srw.StatusCode)
 		labels := map[string]string{"code": status, "method": r.Method}
 		// Update requests counter metric
 		metricRequestsTotalCounter.With(labels).Inc()
+		// Update request latency metric
+		elapsed := tock.Sub(tick)
+		metricRequestsLatencyNanoSeconds.With(labels).Observe(elapsed.Seconds())
 	})
 }