code: func (s *Server) GRPCServer(certFile, keyFile string) (*grpc.Server, error) {
  	opts := []grpc.ServerOption{
  		grpc.MaxConcurrentStreams(2048),
  	}
  	if certFile != "" && keyFile != "" {
  		creds, err := credentials.NewServerTLSFromFile(certFile, keyFile)
  		if err != nil {
  			return nil, err
  		}
  		opts = append(opts, grpc.Creds(creds))
  	}
  	srv := grpc.NewServer(opts...)
  	envoydisco.RegisterAggregatedDiscoveryServiceServer(srv, s)
  	envoyauthz.RegisterAuthorizationServer(srv, s)
  	return srv, nil
  }
docs: // GRPCServer returns a server instance that can handle XDS and ext_authz
  // requests.