code: func (c *SDKConfig) TokenSource(ctx context.Context) oauth2.TokenSource {
  	return c.conf.TokenSource(ctx, c.initialToken)
  }
docs: // TokenSource returns an oauth2.TokenSource that retrieve tokens from
  // Google Cloud SDK credentials using the provided context.
  // It will returns the current access token stored in the credentials,
  // and refresh it when it expires, but it won't update the credentials
  // with the new access token.