etcdserver: add "etcd_server_id"

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

etcdserver/metrics.go

@@ -110,6 +110,13 @@ var (
 		Help:      "Which Go version server is running with. 1 for 'server_go_version' label with current version.",
 	},
 		[]string{"server_go_version"})
+	serverID = prometheus.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: "etcd",
+		Subsystem: "server",
+		Name:      "id",
+		Help:      "Server or member ID in hexadecimal format. 1 for 'server_id' label with current ID.",
+	},
+		[]string{"server_id"})
 )
 
 func init() {
@@ -127,6 +134,7 @@ func init() {
 	prometheus.MustRegister(quotaBackendBytes)
 	prometheus.MustRegister(currentVersion)
 	prometheus.MustRegister(currentGoVersion)
+	prometheus.MustRegister(serverID)
 
 	currentVersion.With(prometheus.Labels{
 		"server_version": version.Version,

etcdserver/server.go

@@ -54,8 +54,10 @@ import (
 	"github.com/coreos/etcd/store"
 	"github.com/coreos/etcd/version"
 	"github.com/coreos/etcd/wal"
+
 	"github.com/coreos/go-semver/semver"
 	"github.com/coreos/pkg/capnslog"
+	"github.com/prometheus/client_golang/prometheus"
 	"golang.org/x/net/context"
 )
 
@@ -430,6 +432,7 @@ func NewServer(cfg *ServerConfig) (srv *EtcdServer, err error) {
 		reqIDGen:      idutil.NewGenerator(uint16(id), time.Now()),
 		forceVersionC: make(chan struct{}),
 	}
+	serverID.With(prometheus.Labels{"server_id": id.String()}).Set(1)
 
 	srv.applyV2 = &applierV2store{store: srv.store, cluster: srv.cluster}