mvcc/backend: document metrics ExponentialBuckets

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

mvcc/backend/metrics.go

@@ -22,7 +22,10 @@ var (
 		Subsystem: "disk",
 		Name:      "backend_commit_duration_seconds",
 		Help:      "The latency distributions of commit called by backend.",
-		Buckets:   prometheus.ExponentialBuckets(0.001, 2, 14),
+
+		// lowest bucket start of upper bound 0.001 sec (1 ms) with factor 2
+		// highest bucket start of 0.001 sec * 2^13 == 8.192 sec
+		Buckets: prometheus.ExponentialBuckets(0.001, 2, 14),
 	})
 
 	snapshotDurations = prometheus.NewHistogram(prometheus.HistogramOpts{
@@ -30,7 +33,9 @@ var (
 		Subsystem: "disk",
 		Name:      "backend_snapshot_duration_seconds",
 		Help:      "The latency distribution of backend snapshots.",
-		// 10 ms -> 655 seconds
+
+		// lowest bucket start of upper bound 0.01 sec (10 ms) with factor 2
+		// highest bucket start of 0.01 sec * 2^16 == 655.36 sec
 		Buckets: prometheus.ExponentialBuckets(.01, 2, 17),
 	})
 )