Add prometheus endpoints

[matiasilva]

https://pypi.org/project/prometheus-flask-exporter/

[edwinbalani]

As a Timeout dev/sysadmin or LBT deployment manager, I want Flask/webapp level Prometheus metrics available to my monitoring systems, so that I can see how the LBT webapp is handling production loads.

The work on this is done when:

• [ ] the metrics we want to export to begin with are decided
• [ ] those metrics are available at a conventional route (e.g. /metrics) in Prometheus JSON format
• [ ] if the metrics are public, no personal data or private/sensitive information are contained in them
• [ ] if the metrics are to be private, access control is applied properly so that only authorised requests succeed

---

prometheus-flask-exporter looks promising -- from a skim read of its README, it is both highly extensible and has some useful default metrics that it exports, which we could use as a starting point.

Are there any obvious metrics specific to LBT that we want to export from the get-go? Emphasis on 'obvious': given that we'd like this to be ready by Freshers' Fair we have about 24hr to implement and test :cowboy_hat_face:

[edwinbalani]

As I understand things, this can be added in isolation (if we keep the default metrics), so a task suitable for a newcomer