Feature/robust-healt-check

[HugaertsDries]

Introduced a metric endpoint. This endpoint can be integrated within Prometheus by following along with this gist.

[erikap]

Does the metric endpoint add additional information compared to the info we can retrieve from app-http-logger?

[nvdk]

There's some overlap, but this does serve a different purpose. The metric endpoint is useful for real-time metric-based monitoring, such as our prometheus stack.
With this endpoint it's possible to:

• set up a health indicator from docker compose
• sent out real time alerts if delta's are failing
• show the metrics in an admin panel in the application (in my opinion the only reason to not output openmetrics directly, which I'd prefer)

none of which are possible with app http logger

The App http logger empowers analysis and search capabilities of http traffic and a such could give you similar metrics, but more in an after the fact manner.

FYI I'm reworking this PR basing myself on #17 . I'm leaning towards directly exposing metrics in the openmetrics standard vs json though, since I see little value in using a non standard json format for the metrics.