Feature request: add metrics endpoint

[vvanouytsel]

It would be cool if we could add a metrics endpoint that we can scrape with prometheus. I have no clue how difficult would be but let's try to start with exposing a metrics endpoint that exposes the following metric:

• jetspotter_aircraft_spotted

The following labels should be on the metric:

• type
• icao
• registration

If we can get it to work, we can build a dashboard in grafana that displays the aircraft that you spotted. Make it togglable and turn it off by default.

[vvanouytsel]

Closing as it has been added: https://vvanouytsel.github.io/jetspotter/dashboards/