code: func (p *ConnPool) Ping(dc string, addr net.Addr, version int, useTLS bool) (bool, error) {
  	var out struct{}
  	err := p.RPC(dc, addr, version, "Status.Ping", useTLS, struct{}{}, &out)
  	return err == nil, err
  }
docs: // Ping sends a Status.Ping message to the specified server and
  // returns true if healthy, false if an error occurred