Increments any unflushed counter values.

def _flush_batch_incr_counter(self):
        """
        Increments any unflushed counter values.
        """
        for key, count in six.iteritems(self._counter_dict):
            if count == 0:
                continue
            args = list(key) + [count]
            self._incr_counter(*args)
            self._counter_dict[key] = 0