etcdserver: add "etcd_server_id" metric

```
etcd_server_id{server_id="8e9e05c52164694d"} 1
```

Useful for automating membership change operations,
no need to run "endpoint status" or "member list"
command to get member IDs.

Also, useful for "etcd_network" metrics with "To/From" labels.

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

etcdserver/server.go

@@ -60,6 +60,7 @@ import (
 	"github.com/coreos/go-semver/semver"
 	"github.com/coreos/pkg/capnslog"
 	humanize "github.com/dustin/go-humanize"
+	"github.com/prometheus/client_golang/prometheus"
 	"go.uber.org/zap"
 )
 
@@ -511,6 +512,7 @@ func NewServer(cfg ServerConfig) (srv *EtcdServer, err error) {
 		forceVersionC:    make(chan struct{}),
 		AccessController: &AccessController{CORS: cfg.CORS, HostWhitelist: cfg.HostWhitelist},
 	}
+	serverID.With(prometheus.Labels{"server_id": id.String()}).Set(1)
 
 	srv.applyV2 = &applierV2store{store: srv.v2store, cluster: srv.cluster}