Roll out gitlab-metrics-exporter for gitlab-rails

Once we made the new exporter binaries available to the Rails app, we need to roll it out to production:

• Enable and test on staging
• Enable and test in a single production env
• Enable and test in all production envs

This needs to be fleshed out more. I think we should come up with a set of comparison tests we can use to compare metrics exported from the old exporter with the new ones, similar to what we did here.