Serve Prometheus metrics in non-"db serve" process

[philrz]

tl;dr

Right now Prometheus metrics are only made available on http://localhost:9867/metrics if running super db serve. It would be helpful if we had an option to open this up even when running super in regular, non-db serve mode, as this would allow for things like getting per-second memory/heap usage stats when studying vector runtime performance.

Details

At the time this issue is being opened, super is at commit 4084e01.

When I asked the team about this, @mattnibs offered tips the code to add this as a one-off hack, and I've done this successfully on the prometheus-hack branch. I'll keep that branch around in case I or a user should want to apply the commit on a one-off basis. However, if someone could add this as an official flag so it could easily be enabled whenever needed that would be great.