Add Prometheus compatible metrics endpoint

[dsluijk]

Is your feature request related to a problem? Please describe.

For monitoring purposes it might be nice to add a Prometheus compatible /metrics routes which can be consumed by VictoriaMetrics. That way we can extract data about the internal usage of events, and generate nice looking graphs.

Describe the solution you'd like

An new endpoint should be created which exports the most important data generated by events (total tickets sold, currently open tickets, amount of items in cart). As this data is sensitive, and therefore should not be public, I recommend exposing this route on a different HTTP listener (e.g. on a different port). This is common practice for metrics endpoints, and allows VictoriaMetrics running in the cluster to reach the metrics without it being public.

While standard metrics (like memory usage and request count) can also be added, these are mostly covered by other metrics sources. The scope of this feature can therefore be limited to unique metrics of events. There isn't really a list of metrics which would be nice to have, so please take a look for yourself.

Describe alternatives you've considered

None, this feature does currently not exist.

Additional context

N/A.