code: func (h *Client) ClientID(clientID string) *Client {
  	h.clientID = clientID
  	h.clientIDSet = true
  	return h
  }
docs: // This anonymously identifies a particular user, device, or
  // browser instance. For the web, this is generally stored
  // as a first-party cookie with a two-year expiration. For
  // mobile apps, this is randomly generated for each particular
  // instance of an application install. The value of this field
  // should be a random UUID (version 4) as described in http://www.ietf.org/rfc/rfc4122.txt