This MR adds three new [OpenMetrics](https://openmetrics.io/)/[Prometheus](https://prometheus.io/) metrics for [Rack::Attack](https://github.com/rack/rack-attack). RackAttack takes care of **throttling** (and sometimes just tracking) authenticated and unauthenticated requests. We log RackAttack events, but we want to have some metrics for these to have better visibility into our Application rate-limiting mechanism.

This change will enable us to build a better dashboard to troubleshoot rate-limiting and throttling issues. Please see the following issues for more context:

* gitlab-com/gl-infra/production-engineering#25743
* gitlab-com/gl-infra/production-engineering#25767