Is this a new feature request?

• I have searched the existing issues

Wanted change

Add the ability to run ./bin/prometheus_exporter as a service

Reason for change

To expose Prometheus metrics when using multiple Puma processes, a separate process needs to be run to collect the metrics and re-expose them.

https://docs.joinmastodon.org/admin/config/#prometheus

Proposed code change

An environment variable (probably?) to configure starting the ./bin/prometheus_exporter process. Existing configuration can kick in from there.