etcdserver/api/v3rpc: display all registered gRPC metrics at start

Previously, only display the one that has been requested at least once.
Now it shows all metrics, as we do in v3.3 and v3.4+.

grpc_server_started_total{grpc_method="Alarm",grpc_service="etcdserverpb.Maintenance",grpc_type="unary"} 0
grpc_server_started_total{grpc_method="AuthDisable",grpc_service="etcdserverpb.Auth",grpc_type="unary"} 0
grpc_server_started_total{grpc_method="AuthEnable",grpc_service="etcdserverpb.Auth",grpc_type="unary"} 0
grpc_server_started_total{grpc_method="Authenticate",grpc_service="etcdserverpb.Auth",grpc_type="unary"} 0
grpc_server_started_total{grpc_method="Compact",grpc_service="etcdserverpb.KV",grpc_type="unary"} 0
grpc_server_started_total{grpc_method="Defragment",grpc_service="etcdserverpb.Maintenance",grpc_type="unary"} 0
grpc_server_started_total{grpc_method="DeleteRange",grpc_service="etcdserverpb.KV",grpc_type="unary"} 0

Should help document metrics.

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

etcdserver/api/v3rpc/grpc.go

@@ -23,6 +23,8 @@ import (
 
 	"github.com/coreos/etcd/etcdserver"
 	pb "github.com/coreos/etcd/etcdserver/etcdserverpb"
+
+	"github.com/grpc-ecosystem/go-grpc-prometheus"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/credentials"
 	"google.golang.org/grpc/grpclog"
@@ -67,5 +69,9 @@ func Server(s *etcdserver.EtcdServer, tls *tls.Config, gopts ...grpc.ServerOptio
 			grpclog.SetLoggerV2(grpclog.NewLoggerV2(ioutil.Discard, os.Stderr, os.Stderr))
 		}
 	})
+
+	// to display all registered metrics with zero values
+	grpc_prometheus.Register(grpcServer)
+
 	return grpcServer
 }