Return (IsMaster, round_trip_time).

        Can raise ConnectionFailure or OperationFailure.

def _check_with_socket(self, sock_info, metadata=None):
        """Return (IsMaster, round_trip_time).

        Can raise ConnectionFailure or OperationFailure.
        """
        cmd = SON([('ismaster', 1)])
        if metadata is not None:
            cmd['client'] = metadata
        start = _time()
        request_id, msg, max_doc_size = message.query(
            0, 'admin.$cmd', 0, -1, cmd,
            None, DEFAULT_CODEC_OPTIONS)

        # TODO: use sock_info.command()
        sock_info.send_message(msg, max_doc_size)
        raw_response = sock_info.receive_message(1, request_id)
        result = helpers._unpack_response(raw_response)
        return IsMaster(result['data'][0]), _time() - start