WIP: Set fallback prometheus runtime dir

We are currently getting a bunch of warnings from prometheus-client and it falling back to tmp when using rake and rails tasks.

This is because we are only setting the prometheus_multiproc_dir for rails runit services, and not for any other rails script we run.

This is one idea I had, where we set a fallback directory (when we have detected a runtime_dir) for any rails process that hasn't specified its own directory.