Ensure that all spans from the queue are submitted.
        Returns Future that will be completed once the queue is empty.

def close(self):
        """
        Ensure that all spans from the queue are submitted.
        Returns Future that will be completed once the queue is empty.
        """
        with self.stop_lock:
            self.stopped = True

        return ioloop_util.submit(self._flush, io_loop=self.io_loop)