If you haven't flagged it to not count calls to the app, use a filter based on TRANSFER FROM and TRANSFER TO (from AA, to HG). By default apps typically don't count, so you may need to go into CSM programming, find the app ext, and un-check the "don't count as rung" and "don't count as answered". Keep in mind that this may impact ring-time reports, you may need to change how that is calculated.
Another easy way to count is to program a phantom ext to ring the HG, then point the aa key choice to the phantom. Count calls passing through the phantom.