feat: add etcd_debugging_lease_total gauge metric

Signed-off-by: Jimmy- <zhenguo251@gmail.com>

server/lease/lessor.go

@@ -303,6 +303,7 @@ func (le *lessor) Grant(id LeaseID, ttl int64) (*Lease, error) {
 
 	leaseTotalTTLs.Observe(float64(l.ttl))
 	leaseGranted.Inc()
+	leaseActive.Inc()
 
 	if le.isPrimary() {
 		item := &LeaseWithTime{id: l.ID, time: l.expiry}
@@ -351,6 +352,7 @@ func (le *lessor) Revoke(id LeaseID) error {
 	txn.End()
 
 	leaseRevoked.Inc()
+	leaseActive.Dec()
 	return nil
 }
 
@@ -812,6 +814,7 @@ func (le *lessor) initAndRecover() {
 	}
 	le.leaseExpiredNotifier.Init()
 	heap.Init(&le.leaseCheckpointHeap)
+	leaseActive.Set(float64(len(le.leaseMap)))
 
 	le.b.ForceCommit()
 }

server/lease/metrics.go

@@ -49,11 +49,19 @@ var (
 			// 1 second -> 3 months
 			Buckets: prometheus.ExponentialBuckets(1, 2, 24),
 		})
+
+	leaseActive = prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace: "etcd_debugging",
+		Subsystem: "lease",
+		Name:      "active",
+		Help:      "The current number of active leases.",
+	})
 )
 
 func init() {
 	prometheus.MustRegister(leaseGranted)
 	prometheus.MustRegister(leaseRevoked)
 	prometheus.MustRegister(leaseRenewed)
 	prometheus.MustRegister(leaseTotalTTLs)
+	prometheus.MustRegister(leaseActive)
 }