feature: prometheus exporter

[NotAShelf]

• tiny_http
• prometheus_exporter

This feature will be gated way behind a compile-time feature that the user has to opt-in to.

[RGBCube]

Do we need the prometheus_exporter crate? A normal tiny_http server might be simpler to manage.

[NotAShelf]

I have no idea, I need to read into how much heavy liftinig prometheus_exporter does for me and whether I want to do it myself. Question for later.