Add request_duration_seconds metric

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())
 	})
 }