code: func (c *Client) Ping(url string) *PingService {
  	return NewPingService(c).URL(url)
  }
docs: // Ping checks if a given node in a cluster exists and (optionally)
  // returns some basic information about the Elasticsearch server,
  // e.g. the Elasticsearch version number.
  //
  // Notice that you need to specify a URL here explicitly.