Permanently enable external Puma metrics server

This moves us away from running in-process server threads to export metrics. The change was behind an environment variable.

I made some other smaller clean-ups.

I did not add a changelog entry because it is not a user-facing change. It is very much an infrastructure change, but it doesn't have to be administered, it is self-activating, and the ideal outcome is that nothing changes on the surface of things.

Related to #350548 (closed)