Function metricsMiddleware

src/libraries/go/lib/pkg/core/httpservice.go:510–556

(latencyMetricsName string, registerer prometheus.Registerer)

			}

			// Create new logger with fields and pass along
			requestLogger := GetLogger(rootCtx).WithFields(logFields)
			next.ServeHTTP(rw, r.WithContext(WithLogger(r.Context(), requestLogger)))
		})
	}
}

func metricsMiddleware(latencyMetricsName string, registerer prometheus.Registerer) mux.MiddlewareFunc {
	var latency *prometheus.HistogramVec
	var count *prometheus.CounterVec
	if latencyMetricsName != "" {
		latency = prometheus.NewHistogramVec(prometheus.HistogramOpts{
			Name: fmt.Sprintf("%s_http_duration_seconds", latencyMetricsName),
			Help: "Duration of HTTP requests.",
		}, []string{"path", "method"})
		count = prometheus.NewCounterVec(prometheus.CounterOpts{
			Name: fmt.Sprintf("%s_http_request_counts", latencyMetricsName),
			Help: "Counter of HTTP requests.",
		}, []string{"path", "method", "status_code"})

		registerer.MustRegister(latency)
		registerer.MustRegister(count)
	}

	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
			log := GetLogger(r.Context())
			w := getRRW(rw)
			defer func() {
				log.WithField("status_code", w.statusCode).Debugf("Handler finished")
				if count != nil {
					count.WithLabelValues(r.URL.Path, r.Method, fmt.Sprintf("%d", w.statusCode)).Inc()
				}
				// TODO(mcamp) this is a hack to get lightstep to recognize the span
				// as an error. Either lightstep launcher or otelmux (probably the latter) should give us
				// a hook to set this attribute. Until then we'll just set it here.
				if v, _ := semconv.SpanStatusFromHTTPStatusCode(w.statusCode); v == codes.Error {
					trace.SpanFromContext(r.Context()).SetAttributes(attribute.Bool("error", true))
				}
				// Return the writer back to the pool
				putRRW(w)
			}()
			log.Debugf("Handler started")

			if latency != nil {
				timer := prometheus.NewTimer(latency.WithLabelValues(r.URL.Path, r.Method))
				next.ServeHTTP(w.writer, r)
				timer.ObserveDuration()
			} else {
				next.ServeHTTP(w.writer, r)