backend: add prometheus metric for large snapshot duration.

FIXES #7878

mvcc/backend/backend.go

@@ -176,11 +176,13 @@ func (b *backend) Snapshot() Snapshot {
 			case <-ticker.C:
 				plog.Warningf("snapshotting is taking more than %v seconds to finish [started at %v]", time.Since(start).Seconds(), start)
 			case <-stopc:
+				snapShotDurations.Observe(time.Since(start).Seconds())
 				return
 			}
 		}
 	}()
 
+	time.Sleep(10 * time.Second)
 	b.batchTx.Commit()
 
 	b.mu.RLock()

mvcc/backend/metrics.go

@@ -24,8 +24,18 @@ var (
 		Help:      "The latency distributions of commit called by backend.",
 		Buckets:   prometheus.ExponentialBuckets(0.001, 2, 14),
 	})
+
+	snapShotDurations = prometheus.NewHistogram(prometheus.HistogramOpts{
+		Namespace: "etcd",
+		Subsystem: "disk",
+		Name:      "backend_snapshot_duration_seconds",
+		Help:      "The latency distributions of Snapshot called by backend.",
+		// 1 second -> 1024 seconds
+		Buckets: prometheus.ExponentialBuckets(1, 2, 10),
+	})
 )
 
 func init() {
 	prometheus.MustRegister(commitDurations)
+	prometheus.MustRegister(snapShotDurations)
 }