Add Prometheus Support

[jcarvalho]

The introduction of the Frontend in 1.15.0 brought web support to the project, which would be a great opportunity to expose a Prometheus Metrics endpoint, containing information on the current state of the various registered devices (state, LQI, battery, etc).

If there's interest, I would be glad to work on this 🙂 .

Describe alternatives you've considered

An alternative to this endpoint would be to integrate with another software (such as Home Assistant), and scrape the Prometheus metrics from there (this is the approach I'm currently taking).