Add prometheus metrics for the core service http server

[olevski]

Currently we export metrics only from the redis queue.

But there are no metrics on the requests and response times form the core service. It would be nice to have this.

This is a relevant guide on how we can achieve this: https://scribe.nixnet.services/@damianmyerscough/monitoring-gunicorn-with-prometheus-789954150069

Other useful links:

• https://grafana.com/grafana/dashboards/16163-gunicorn-monitoring/
• https://github.com/Blueswen/gunicorn-monitoring

[Panaetius]

check if we can use ingress logs for this instead, might be easier