etcdserver: add "etcd_server_snapshot_apply_inflights_total"

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

etcdserver/metrics.go

@@ -76,6 +76,12 @@ var (
 		Name:      "slow_apply_total",
 		Help:      "The total number of slow apply requests (likely overloaded from slow disk).",
 	})
+	applySnapshotInProgress = prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace: "etcd",
+		Subsystem: "server",
+		Name:      "snapshot_apply_in_progress_total",
+		Help:      "1 if the server is applying the incoming snapshot. 0 if none.",
+	})
 	proposalsCommitted = prometheus.NewGauge(prometheus.GaugeOpts{
 		Namespace: "etcd",
 		Subsystem: "server",
@@ -153,6 +159,7 @@ func init() {
 	prometheus.MustRegister(leaderChanges)
 	prometheus.MustRegister(heartbeatSendFailures)
 	prometheus.MustRegister(slowApplies)
+	prometheus.MustRegister(applySnapshotInProgress)
 	prometheus.MustRegister(proposalsCommitted)
 	prometheus.MustRegister(proposalsApplied)
 	prometheus.MustRegister(proposalsPending)

etcdserver/server.go

@@ -1113,6 +1113,7 @@ func (s *EtcdServer) applySnapshot(ep *etcdProgress, apply *apply) {
 	if raft.IsEmptySnap(apply.snapshot) {
 		return
 	}
+	applySnapshotInProgress.Inc()
 
 	lg := s.getLogger()
 	if lg != nil {
@@ -1138,6 +1139,7 @@ func (s *EtcdServer) applySnapshot(ep *etcdProgress, apply *apply) {
 		} else {
 			plog.Infof("finished applying incoming snapshot at index %d", ep.snapi)
 		}
+		applySnapshotInProgress.Dec()
 	}()
 
 	if apply.snapshot.Metadata.Index <= ep.appliedi {