server: fix learner metric incorrect issue

Signed-off-by: YaoC <chengyao09@hotmail.com>

server/etcdserver/server.go

@@ -335,6 +335,7 @@ func NewServer(cfg config.ServerConfig) (srv *EtcdServer, err error) {
 		firstCommitInTerm:     notify.NewNotifier(),
 		clusterVersionChanged: notify.NewNotifier(),
 	}
+	srv.buildLearnerMetric()
 	serverID.With(prometheus.Labels{"server_id": b.cluster.nodeID.String()}).Set(1)
 	srv.cluster.SetVersionChangedNotifier(srv.clusterVersionChanged)
 
@@ -1085,6 +1086,8 @@ func (s *EtcdServer) applySnapshot(ep *etcdProgress, toApply *toApply) {
 
 	s.cluster.Recover(api.UpdateCapability)
 
+	s.buildLearnerMetric()
+
 	lg.Info("restored cluster configuration")
 	lg.Info("removing old peers from network")
 
@@ -2465,3 +2468,16 @@ func (s *EtcdServer) getTxPostLockInsideApplyHook() func() {
 func (s *EtcdServer) CorruptionChecker() CorruptionChecker {
 	return s.corruptionChecker
 }
+
+func (s *EtcdServer) buildLearnerMetric() {
+	for _, m := range s.cluster.Members() {
+		if m.ID == s.MemberId() {
+			if m.IsLearner {
+				isLearner.Set(1)
+			} else {
+				isLearner.Set(0)
+			}	
+			break
+		}
+	}
+}