Migrate to the official Prometheus client library for ruby

In our ruby monolith, we're using our own fork of the prometheus client library. We needed to do this, because we were running unicorn and needed to keep track of metrics from multiple processes, that could be scraped from a single host.

Since then, we've migrated to puma, but I suspect we still run multiple processes side by side on VMs. The updated official client library now supports multiprocess datastores though we'd still need to check if that works multithreaded+multiprocess.

We should consider migrating to the official client library. Though there's some things that we might want to take care of before we do that, the method arguments and metric initialization has changed since we've forked.