Add /metrics endpoint

We will be adding a AUR exporter soonish (infrastructure!375 (merged)) which only expose the frontpage statistics and works by just scrapping aur.archlinux.org. Ideally aurweb should expose a /metrics endpoint (perhaps restricted)

I think it make sense to postpone this until the Python port is live.

Prometheus Python client: https://github.com/prometheus/client_python

Python /metrics (todo)

• https://pypi.org/project/prometheus-fastapi-instrumentator/
• api_requests_count
  • A single count of HTTP requests destined for /rpc
  • Includes "type" label (extracted query argument) as long as it is included in whitelisted RPC types.

Please coordinate with the DevOps team.