Set up prometheus metrics

Prometheus metrics are important for deployments on clusters so you have some kind of insight in what's going on the service.

For fastapi, the starlette_exporter can be used: https://github.com/stephenhillier/starlette_exporter

Here's an example eox service which uses it

i suggest that anyone other than @mallingerb sets this up end to end (from adding the /metrics endpoint up to seeing the graphs in prometheus) in order to spread awareness of this tool.