code: func (s *Service) PubSubSubscriberClient(c context.Context, projectID string) (*pubsub.Client, error) {
  	ts, err := serverAuth.GetTokenSource(
  		c, serverAuth.AsSelf,
  		serverAuth.WithScopes(gcps.SubscriberScopes...))
  	if err != nil {
  		return nil, err
  	}
  	return pubsub.NewClient(c, projectID,
  		option.WithUserAgent(s.getUserAgent()),
  		option.WithTokenSource(ts))
  }
docs: // PubSubSubscriberClient returns a Pub/Sub client instance that is
  // authenticated with Pub/Sub subscriber scopes.