feat: Enable Prometheus monitoring endpoint

This commit implements the django-prometheus package into donate-neo. The particular intent of this commit is to provide a barebones metrics endpoint as a starting point from which to discuss with the folks who rely upon that monitoring of those metrics how best to expand upon this basic implementation.

Ergo, this is a very basic no-assumptions implementation - no tweaks to default settings have been made, and, beyond setting up database monitoring, no further specific monitoring has been set up. Further, the library which will be used to provide custom metrics (https://github.com/prometheus/client_python) has not yet been added as a dependency in pyproject.toml. These changes will be made in future commits as monitoring needs dictate.

Monitoring metrics added by this commit are available at the new URL endpoint /metrics.