code: func (c *Subcommand) ConfigService(ctx context.Context, host string) (*config.Service, error) {
  	authOpts, err := c.authFlags.Options()
  	if err != nil {
  		return nil, err
  	}
  	client, err := auth.NewAuthenticator(ctx, auth.SilentLogin, authOpts).Client()
  	if err != nil {
  		return nil, err
  	}
  
  	svc, err := config.New(client)
  	if err != nil {
  		return nil, err
  	}
  	svc.BasePath = fmt.Sprintf("https://%s/_ah/api/config/v1/", host)
  	svc.UserAgent = lucicfg.UserAgent
  	return svc, nil
  }
docs: // ConfigService returns a wrapper around LUCI Config API.
  //
  // It is ready for making authenticated RPCs. 'host' is a hostname of the
  // service to hit, e.g. "luci-config.appspot.com".