API metrics - HTTP response code counts (#1600)

pkg/api/metrics.go

@@ -5,6 +5,7 @@
 package api
 
 import (
+	"fmt"
 	"net/http"
 	"time"
 
@@ -16,9 +17,10 @@ type metrics struct {
 	// all metrics fields must be exported
 	// to be able to return them by Metrics()
 	// using reflection
-	RequestCount     prometheus.Counter
-	ResponseDuration prometheus.Histogram
-	PingRequestCount prometheus.Counter
+	RequestCount       prometheus.Counter
+	ResponseDuration   prometheus.Histogram
+	PingRequestCount   prometheus.Counter
+	ResponseCodeCounts *prometheus.CounterVec
 }
 
 func newMetrics() metrics {
@@ -38,6 +40,15 @@ func newMetrics() metrics {
 			Help:      "Histogram of API response durations.",
 			Buckets:   []float64{0.01, 0.1, 0.25, 0.5, 1, 2.5, 5, 10},
 		}),
+		ResponseCodeCounts: prometheus.NewCounterVec(
+			prometheus.CounterOpts{
+				Namespace: m.Namespace,
+				Subsystem: subsystem,
+				Name:      "response_code_count",
+				Help:      "Response count grouped by status code",
+			},
+			[]string{"code", "method"},
+		),
 	}
 }
 
@@ -53,3 +64,50 @@ func (s *server) pageviewMetricsHandler(h http.Handler) http.Handler {
 		s.metrics.ResponseDuration.Observe(time.Since(start).Seconds())
 	})
 }
+
+func (s *server) responseCodeMetricsHandler(h http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		wrapper := newResponseWriter(w)
+		h.ServeHTTP(wrapper, r)
+		s.metrics.ResponseCodeCounts.WithLabelValues(
+			fmt.Sprintf("%d", wrapper.statusCode),
+			r.Method,
+		).Inc()
+	})
+}
+
+// UpgradedResponseWriter adds more functionality on top of ResponseWriter
+type UpgradedResponseWriter interface {
+	http.ResponseWriter
+	http.Pusher
+	http.Hijacker
+	http.Flusher
+	// staticcheck SA1019 CloseNotifier interface is required by gorilla compress handler
+	// nolint:staticcheck
+	http.CloseNotifier // skipcq: SCC-SA1019
+}
+
+type responseWriter struct {
+	UpgradedResponseWriter
+	statusCode  int
+	wroteHeader bool
+}
+
+func newResponseWriter(w http.ResponseWriter) *responseWriter {
+	// StatusOK is called by default if nothing else is called
+	uw := w.(UpgradedResponseWriter)
+	return &responseWriter{uw, http.StatusOK, false}
+}
+
+func (rw *responseWriter) Status() int {
+	return rw.statusCode
+}
+
+func (rw *responseWriter) WriteHeader(code int) {
+	if rw.wroteHeader {
+		return
+	}
+	rw.statusCode = code
+	rw.UpgradedResponseWriter.WriteHeader(code)
+	rw.wroteHeader = true
+}

pkg/api/router.go

@@ -177,6 +177,7 @@ func (s *server) setupRouting() {
 		httpaccess.NewHTTPAccessLogHandler(s.logger, logrus.InfoLevel, s.tracer, "api access"),
 		handlers.CompressHandler,
 		// todo: add recovery handler
+		s.responseCodeMetricsHandler,
 		s.pageviewMetricsHandler,
 		func(h http.Handler) http.Handler {
 			return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {