For each limit in the specified service (or all services if
        ``service`` is ``None``), query the AWS API via ``boto3``
        and find the current usage amounts for that limit.

        This method updates the ``current_usage`` attribute of the
        :py:class:`~.AwsLimit` objects for each service, which can
        then be queried using :py:meth:`~.get_limits`.

        :param service: list of :py:class:`~._AwsService` name(s), or ``None``
          to check all services.
        :type service: :py:obj:`None`, or :py:obj:`list` service names to get
        :param use_ta: check Trusted Advisor for information on limits
        :type use_ta: bool

def find_usage(self, service=None, use_ta=True):
        """
        For each limit in the specified service (or all services if
        ``service`` is ``None``), query the AWS API via ``boto3``
        and find the current usage amounts for that limit.

        This method updates the ``current_usage`` attribute of the
        :py:class:`~.AwsLimit` objects for each service, which can
        then be queried using :py:meth:`~.get_limits`.

        :param service: list of :py:class:`~._AwsService` name(s), or ``None``
          to check all services.
        :type service: :py:obj:`None`, or :py:obj:`list` service names to get
        :param use_ta: check Trusted Advisor for information on limits
        :type use_ta: bool
        """
        to_get = self.services
        if service is not None:
            to_get = dict((each, self.services[each]) for each in service)
        if use_ta:
            self.ta.update_limits()
        for cls in to_get.values():
            if hasattr(cls, '_update_limits_from_api'):
                cls._update_limits_from_api()
            logger.debug("Finding usage for service: %s", cls.service_name)
            cls.find_usage()