code: func makeIAMTokenProvider(ctx context.Context, opts *Options) (internal.TokenProvider, error) {
  	if opts.testingIAMTokenProvider != nil {
  		return opts.testingIAMTokenProvider, nil
  	}
  	return internal.NewIAMTokenProvider(
  		ctx,
  		opts.ActAsServiceAccount,
  		opts.Scopes,
  		opts.Transport)
  }
docs: // makeIAMTokenProvider create TokenProvider to use in Actor mode.
  //
  // Called by ensureInitialized in actor mode.