feat(go): change in-flight metric to only count expensive requests (#845)

Author: Proximyst

pkg/api/middleware/metrics.go

@@ -12,7 +12,7 @@ import (
 var (
 	MetricRequestsInFlight = prometheus.NewGauge(prometheus.GaugeOpts{
 		Name: "http_requests_in_flight",
-		Help: "How many requests are in flight?",
+		Help: "How many expensive requests are in flight?",
 	})
 	MetricRequestDurations = prometheus.NewHistogramVec(prometheus.HistogramOpts{
 		Name: "http_request_duration",
@@ -39,14 +39,25 @@ func RequestMetrics(h http.Handler) http.Handler {
 		r = r.WithContext(ctx)
 
 		now := time.Now()
-		MetricRequestsInFlight.Inc()
 		recorder := &statusRecordingResponseWriter{rw: w}
 		h.ServeHTTP(recorder, r)
-		MetricRequestsInFlight.Dec()
 		MetricRequestDurations.WithLabelValues(r.Method, r.Pattern, strconv.Itoa(recorder.status)).Observe(time.Since(now).Seconds())
 	})
 }
 
+func InFlightMetrics(h http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		tracer := tracer(r.Context())
+		ctx, span := tracer.Start(r.Context(), "InFlightMetrics")
+		defer span.End()
+		r = r.WithContext(ctx)
+
+		MetricRequestsInFlight.Inc()
+		defer MetricRequestsInFlight.Dec() // defer to run despite panics
+		h.ServeHTTP(w, r)
+	})
+}
+
 var (
 	_ http.ResponseWriter = (*statusRecordingResponseWriter)(nil)
 	_ http.Flusher        = (*statusRecordingResponseWriter)(nil)

pkg/api/mux.go

@@ -35,8 +35,20 @@ func NewHandler(
 	mux.Handle("GET /metrics", middleware.TracingFor("promhttp.HandlerFor", promhttp.HandlerFor(metrics, promhttp.HandlerOpts{Registry: metrics})))
 	mux.Handle("GET /healthz", HandleGetHealthz())
 	mux.Handle("GET /version", HandleGetVersion(versions, browser))
-	mux.Handle("GET /render", middleware.RequireAuthToken(middleware.TrustedURL(limiter.Limit(HandleGetRender(browser))), serverConfig.AuthTokens...))
-	mux.Handle("GET /render/csv", middleware.RequireAuthToken(middleware.TrustedURL(limiter.Limit(HandleGetRenderCSV(browser))), serverConfig.AuthTokens...))
+	mux.Handle("GET /render",
+		middleware.RequireAuthToken(
+			middleware.TrustedURL(
+				limiter.Limit(
+					middleware.InFlightMetrics(
+						HandleGetRender(browser)))),
+			serverConfig.AuthTokens...))
+	mux.Handle("GET /render/csv",
+		middleware.RequireAuthToken(
+			middleware.TrustedURL(
+				limiter.Limit(
+					middleware.InFlightMetrics(
+						HandleGetRenderCSV(browser)))),
+			serverConfig.AuthTokens...))
 	mux.Handle("GET /render/version", HandleGetRenderVersion(versions))
 
 	handler := middleware.RequestMetrics(mux)