instrumented gate: observe permitted and non-permitted queries separately

CHANGELOG.md

@@ -204,6 +204,7 @@
   * `gate_duration_seconds`
   * `kv_request_duration_seconds`
   * `operation_duration_seconds`
+* [ENHANCEMENT] Add `outcome` label to `gate_duration_seconds` metric. Possible values are `rejected_canceled`, `rejected_deadline_exceeded`, `rejected_other`, and `permitted`. #512
 * [BUGFIX] spanlogger: Support multiple tenant IDs. #59
 * [BUGFIX] Memberlist: fixed corrupted packets when sending compound messages with more than 255 messages or messages bigger than 64KB. #85
 * [BUGFIX] Ring: `ring_member_ownership_percent` and `ring_tokens_owned` metrics are not updated on scale down. #109

gate/gate.go

@@ -64,15 +64,15 @@ func NewInstrumented(reg prometheus.Registerer, maxConcurrent int, gate Gate) Ga
 			Name: "gate_queries_in_flight",
 			Help: "Number of queries that are currently in flight.",
 		}),
-		duration: promauto.With(reg).NewHistogram(prometheus.HistogramOpts{
+		duration: promauto.With(reg).NewHistogramVec(prometheus.HistogramOpts{
 			Name:    "gate_duration_seconds",
 			Help:    "How many seconds it took for queries to wait at the gate.",
 			Buckets: []float64{0.01, 0.1, 0.3, 0.6, 1, 3, 6, 9, 20, 30, 60, 90, 120, 240, 360, 720},
 			// Use defaults recommended by Prometheus for native histograms.
 			NativeHistogramBucketFactor:     1.1,
 			NativeHistogramMaxBucketNumber:  100,
 			NativeHistogramMinResetDuration: time.Hour,
-		}),
+		}, []string{"outcome"}),
 	}
 
 	g.max.Set(float64(maxConcurrent))
@@ -84,20 +84,28 @@ type instrumentedGate struct {
 
 	max      prometheus.Gauge
 	inflight prometheus.Gauge
-	duration prometheus.Histogram
+	duration *prometheus.HistogramVec
 }
 
 func (g *instrumentedGate) Start(ctx context.Context) error {
 	start := time.Now()
-	defer func() {
-		g.duration.Observe(time.Since(start).Seconds())
-	}()
 
 	err := g.gate.Start(ctx)
 	if err != nil {
+		var reason string
+		switch {
+		case errors.Is(err, context.Canceled):
+			reason = "rejected_canceled"
+		case errors.Is(err, context.DeadlineExceeded):
+			reason = "rejected_deadline_exceeded"
+		default:
+			reason = "rejected_other"
+		}
+		g.duration.WithLabelValues(reason).Observe(time.Since(start).Seconds())
 		return err
 	}
 
+	g.duration.WithLabelValues("permitted").Observe(time.Since(start).Seconds())
 	g.inflight.Inc()
 	return nil
 }