code: func (h *Client) UserID(userID string) *Client {
  	h.userID = userID
  	h.userIDSet = true
  	return h
  }
docs: // This is intended to be a known identifier for a user provided
  // by the site owner/tracking library user. It may not itself
  // be PII (personally identifiable information). The value
  // should never be persisted in GA cookies or other Analytics
  // provided storage.