code: func NewServiceAccountTokenProvider(ctx context.Context, jsonKey []byte, path string, scopes []string) (TokenProvider, error) {
  	return &serviceAccountTokenProvider{
  		ctx:     ctx,
  		jsonKey: jsonKey,
  		path:    path,
  		scopes:  scopes,
  	}, nil
  }
docs: // NewServiceAccountTokenProvider returns TokenProvider that uses service
  // account private key (on disk or in memory) to make access tokens.