fix(metrics): separate duration histogram by success/error result

Author: robertodauria

handler/handler.go

@@ -97,14 +97,16 @@ func (h *Handler) exchangeToken(ctx context.Context, apiKey string) (string, err
 
 	start := time.Now()
 	resp, err := h.config.HTTPClient.Do(req)
-	metrics.UpstreamRequestDuration.Observe(time.Since(start).Seconds())
+	duration := time.Since(start).Seconds()
 
 	if err != nil {
+		metrics.UpstreamRequestDuration.WithLabelValues("error").Observe(duration)
 		metrics.UpstreamRequestsTotal.WithLabelValues("error").Inc()
 		return "", fmt.Errorf("failed to send request: %w", err)
 	}
 	defer resp.Body.Close()
 
+	metrics.UpstreamRequestDuration.WithLabelValues("success").Observe(duration)
 	metrics.UpstreamRequestsTotal.WithLabelValues(strconv.Itoa(resp.StatusCode)).Inc()
 
 	if resp.StatusCode != http.StatusOK {

metrics/metrics.go

@@ -37,11 +37,12 @@ var (
 	)
 
 	// UpstreamRequestDuration measures upstream token exchange latency.
-	UpstreamRequestDuration = promauto.NewHistogram(
+	UpstreamRequestDuration = promauto.NewHistogramVec(
 		prometheus.HistogramOpts{
 			Name:    "speedproxy_upstream_request_duration_seconds",
 			Help:    "Duration of upstream token exchange requests.",
 			Buckets: prometheus.DefBuckets,
 		},
+		[]string{"result"},
 	)
 )