code: func DefaultClient(ctx context.Context, scope ...string) (*http.Client, error) {
  	ts, err := DefaultTokenSource(ctx, scope...)
  	if err != nil {
  		return nil, err
  	}
  	return oauth2.NewClient(ctx, ts), nil
  }
docs: // DefaultClient returns an HTTP Client that uses the
  // DefaultTokenSource to obtain authentication credentials.
  //
  // This client should be used when developing services
  // that run on Google App Engine or Google Compute Engine
  // and use "Application Default Credentials."
  //
  // For more details, see:
  // https://developers.google.com/accounts/docs/application-default-credentials
  //