etcdserver/*: add "etcd_cluster_version" metric

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

etcdserver/api/membership/cluster.go

@@ -36,6 +36,7 @@ import (
 	"go.etcd.io/etcd/version"
 
 	"github.com/coreos/go-semver/semver"
+	"github.com/prometheus/client_golang/prometheus"
 	"go.uber.org/zap"
 )
 
@@ -500,6 +501,7 @@ func (c *RaftCluster) SetVersion(ver *semver.Version, onSet func(*zap.Logger, *s
 	if c.be != nil {
 		mustSaveClusterVersionToBackend(c.be, ver)
 	}
+	ClusterVersionMetrics.With(prometheus.Labels{"cluster_version": ver.String()}).Set(1)
 	onSet(c.lg, ver)
 }
 

etcdserver/api/membership/metrics.go

@@ -0,0 +1,31 @@
+// Copyright 2018 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package membership
+
+import "github.com/prometheus/client_golang/prometheus"
+
+var (
+	ClusterVersionMetrics = prometheus.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: "etcd",
+		Subsystem: "cluster",
+		Name:      "version",
+		Help:      "Which version is running. 1 for 'cluster_version' label with current cluster version",
+	},
+		[]string{"cluster_version"})
+)
+
+func init() {
+	prometheus.MustRegister(ClusterVersionMetrics)
+}

etcdserver/server.go

@@ -768,6 +768,7 @@ func (s *EtcdServer) start() {
 		} else {
 			plog.Infof("starting server... [version: %v, cluster version: %v]", version.Version, version.Cluster(s.ClusterVersion().String()))
 		}
+		membership.ClusterVersionMetrics.With(prometheus.Labels{"cluster_version": s.ClusterVersion().String()}).Set(1)
 	} else {
 		if lg != nil {
 			lg.Info(