feat: support grpc balancer health check

[shenqidebaozi]

When service name is not passed, gRPC health server will set a service with null key

// NewServer returns a new Server.
func NewServer() *Server {
	return &Server{
		statusMap: map[string]healthpb.HealthCheckResponse_ServingStatus{"": healthpb.HealthCheckResponse_SERVING},
		updates:   make(map[string]map[healthgrpc.Health_WatchServer]chan healthpb.HealthCheckResponse_ServingStatus),
	}
}

At the same time, if the error returned by the peer service is that the healthcheck is not implemented, the connection status is also available. gRPC believes that the connection is available as long as the health check is passed or the error returned is not implemented

		for {
			err = s.RecvMsg(resp)

			// Reports healthy for the LBing purposes if health check is not implemented in the server.
			if status.Code(err) == codes.Unimplemented {
				setConnectivityState(connectivity.Ready, nil)
				return err
			}

			// Reports unhealthy if server's Watch method gives an error other than UNIMPLEMENTED.
			if err != nil {
				setConnectivityState(connectivity.TransientFailure, fmt.Errorf("connection active but received health check RPC error: %v", err))
				continue retryConnection
			}

			// As a message has been received, removes the need for backoff for the next retry by resetting the try count.
			tryCnt = 0
			if resp.Status == healthpb.HealthCheckResponse_SERVING {
				setConnectivityState(connectivity.Ready, nil)
			} else {
				setConnectivityState(connectivity.TransientFailure, fmt.Errorf("connection active but health check failed. status=%s", resp.Status))
			}
		}