spawn prometheus exporter

[augustas1]

What does this PR do?

Spawns Prometheus exporter in the background. Currently metrics are not accessible via PROMETHEUS_PORT (default 9000), only via main port /metrics route.

Code addition matches the same what PrometheusBuilder::install does.

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@alvarobartt @Narsil

[alvarobartt]

Hey @augustas1, thanks for your contribution! Indeed this seems related to #729, but AFAIK the Prometheus port only applies for gRPC, as it requires to deploy the endpoint over HTTP; whereas on HTTP is not required as we can simply create the /metrics route within the default port.

Could you elaborate on why do you think this is required or why? I guess it can be misleading to expose a --prometheus-port argument and then not being "used" over HTTP for querying the /metrics endpoint but AFAIK the change should be on the argument description or on the docs rather than on the behaviour on whether Prometheus is spawned or not within the HTTP server, thoughts?

Thanks in advance 🤗