exception metrics never trigger

[anarcat]

In our monitoring, we're trying to alert when django raises (too many) exceptions. But it seems the metric seemingly designed for this (django_http_requests_exceptions_total) never actually increments here, even though we can raise exceptions.

It seems the process_exception method in PrometheusAfterMiddleware should handle that automatically, but doesn't.

We're running Django 4.2.15 on Debian 12 bookworm ("stable"). Our downstream bug report is https://gitlab.torproject.org/tpo/web/donate-neo/-/issues/115

Thanks!