Add Prometheus metrics

[Robin5605]

Add a metrics endpoint which can be scraped by Prometheus. The follow metrics can be scraped by Prometheus:

• Packages ingested (queued) since API startup
• Total packages scanned sinced API startup (success + failure)
• Total packages scanned since API startup that were successful
• Total packages scanned since API startup that failed
• Total packages reported since API startup
• Average request time
• Total number of requests served

[AbooMinister25]

Everything LGTM

[Xithrius]

I can continue this PR in a new one with prometheus-fastapi-instrumentator and prometheus-client, and then add Grafana to the infra of this org somewhere. Thoughts @Robin5605?

[Robin5605]

@Xithrius sounds great! I'll be closing this issue then as it's stale.

[Robin5605]

Hi @Xithrius , I actually think we can just continue work on this one - I feel like working on this again and will be pushing some commits here shortly. Feel free to also push commits and make changes to this branch if you so please. Just didn't want 2 PRs that did essentially the same thing and we've got stuff to work off here already

[Robin5605]

Closing for now as #257 gives us observability. May open in the future if we want to move away from logfire.