etcdserver: add learner metrics

docs/metrics/latest

@@ -533,6 +533,21 @@ etcd_server_is_leader
 # type: "counter"
 etcd_server_leader_changes_seen_total
 
+# name: "etcd_server_is_learner"
+# description: "Whether or not this member is a learner. 1 if is, 0 otherwise."
+# type: "gauge"
+etcd_server_is_learner
+
+# name: "etcd_server_learner_promote_failures"
+# description: "The total number of learner promote  failures (likely learner not ready)."
+# type: "counter"
+etcd_server_learner_promote_failures
+
+# name: "etcd_server_learner_promote_successes"
+# description: "The total number of successful learner promotions."
+# type: "counter"
+etcd_server_learner_promote_successes
+
 # name: "etcd_server_proposals_applied_total"
 # description: "The total number of consensus proposals applied."
 # type: "gauge"

etcdserver/metrics.go

@@ -44,6 +44,20 @@ var (
 		Name:      "leader_changes_seen_total",
 		Help:      "The number of leader changes seen.",
 	})
+	learnerPromoteFailed = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "etcd",
+		Subsystem: "server",
+		Name:      "learner_promote_failures",
+		Help:      "The total number of learner promote failures (likely learner not ready).",
+	},
+		[]string{"Reason"},
+	)
+	learnerPromoteSucceed = prometheus.NewCounter(prometheus.CounterOpts{
+		Namespace: "etcd",
+		Subsystem: "server",
+		Name:      "learner_promote_successes",
+		Help:      "The total number of successful learner promotions.",
+	})
 	heartbeatSendFailures = prometheus.NewCounter(prometheus.CounterOpts{
 		Namespace: "etcd",
 		Subsystem: "server",

etcdserver/server.go

@@ -1646,7 +1646,11 @@ func (s *EtcdServer) PromoteMember(ctx context.Context, id uint64) ([]*membershi
 	// fails with ErrNotLeader, forward the request to leader node via HTTP. If promoteMember call fails with error
 	// other than ErrNotLeader, return the error.
 	resp, err := s.promoteMember(ctx, id)
+	if err == nil {
+		learnerPromoteSuceed.Inc()
+	}
 	if err != ErrNotLeader {
+		learnerPromoteFailed.WithLabelValues(err.Error()).Inc()
 		return resp, err
 	}