detect CloseNotify capability in accesslog and metrics

pkg/middlewares/metrics/metrics.go

@@ -89,10 +89,10 @@ func (m *metricsMiddleware) ServeHTTP(rw http.ResponseWriter, req *http.Request)
 	}(labels)
 
 	start := time.Now()
-	recorder := &responseRecorder{rw, http.StatusOK}
+	recorder := newResponseRecorder(rw)
 	m.next.ServeHTTP(recorder, req)
 
-	labels = append(labels, "code", strconv.Itoa(recorder.statusCode))
+	labels = append(labels, "code", strconv.Itoa(recorder.getCode()))
 	m.reqsCounter.With(labels...).Add(1)
 	m.reqDurationHistogram.With(labels...).Observe(time.Since(start).Seconds())
 }

pkg/middlewares/metrics/recorder.go

@@ -6,6 +6,23 @@ import (
 	"net/http"
 )
 
+type recorder interface {
+	http.ResponseWriter
+	http.Flusher
+	getCode() int
+}
+
+func newResponseRecorder(rw http.ResponseWriter) recorder {
+	rec := &responseRecorder{
+		ResponseWriter: rw,
+		statusCode:     http.StatusOK,
+	}
+	if _, ok := rw.(http.CloseNotifier); !ok {
+		return rec
+	}
+	return responseRecorderWithCloseNotify{rec}
+}
+
 // responseRecorder captures information from the response and preserves it for
 // later analysis.
 type responseRecorder struct {
@@ -13,6 +30,20 @@ type responseRecorder struct {
 	statusCode int
 }
 
+type responseRecorderWithCloseNotify struct {
+	*responseRecorder
+}
+
+// CloseNotify returns a channel that receives at most a
+// single value (true) when the client connection has gone away.
+func (r *responseRecorderWithCloseNotify) CloseNotify() <-chan bool {
+	return r.ResponseWriter.(http.CloseNotifier).CloseNotify()
+}
+
+func (r *responseRecorder) getCode() int {
+	return r.statusCode
+}
+
 // WriteHeader captures the status code for later retrieval.
 func (r *responseRecorder) WriteHeader(status int) {
 	r.ResponseWriter.WriteHeader(status)