code: func (s *Server) Check(ctx context.Context, in *api.HealthCheckRequest) (*api.HealthCheckResponse, error) {
  	s.mu.Lock()
  	defer s.mu.Unlock()
  	if in.Service == "" {
  		// check the server overall health status.
  		return &api.HealthCheckResponse{
  			Status: api.HealthCheckResponse_SERVING,
  		}, nil
  	}
  	if status, ok := s.statusMap[in.Service]; ok {
  		return &api.HealthCheckResponse{
  			Status: status,
  		}, nil
  	}
  	return nil, status.Errorf(codes.NotFound, "unknown service")
  }
docs: // Check checks if the grpc server is healthy and running.