[Feature] Stats exporter for Prometheus

Hi 👋

My project is to integrate an stats exporter for prometheus. In short, a few features:
- on/off via admin
- endpoint protection for admins only
- export of challenges stats, scorebord, admin metrics, ...
- ability to add custom metrics for plugins

Basically, I was thinking of making a plug-in, but I think it would be a good addition to the project directly.
So before starting development, I wanted to have a first feadback :)

Start by clarifying whether this belongs in a plug-in or the core project, then define the required admin controls and protected endpoint. Scope the challenge, scoreboard, admin, and custom-plugin metrics before implementation; done should mean the requirements and integration point are agreed.

Written by the indexing model from the issue text.