chore: abstract the rails_queueing SLI out in a function

chore: abstract the rails_queueing SLI out in a function

This will allow us to reuse it for the different webservices for GitLab.com.

This causes no diff in the generated files for the GET-hybred architectures

feat(metrics-catalog): add experimental to SLIs

This allows defining an SLI as experimental. An experimental SLI will generate all of the recording rules, but no alerts.

This allows us to use the analysis dashboards to determine the right SLO for an SLI without having to silence alerts or reduce the severity.

This illustrates how this works by marking the rails_queueing SLI as experimental and removing the alerts for that SLI from the GET-hybrid webservice.

The SLI was initially added to the GET-hybrid in !8684 (merged). But when discussing this on Slack, we decided that we'd rather not alert on it in the beginning.

feat(new-sli): this adds the rails_queueing SLI for GitLab.com

This adds the rails_queuing SLI for GitLab.com. This SLI shows how long a request waits in workhorse before it can be picked up by a puma worker.

It sets SLOs based on what is currently available in metrics, most services set a 99.5% SLO. We will not alert on these SLIs at all for now, until we can validate the metrics.