Ship dashboard to display Service Level Indicators (SLIs)

Summary

Related to #4374 (closed). Once we have recording rules for SLIs we should ship a Grafana dashboard to display this data.

Proposal

In order for users and support to be able to quickly determine the health of a GitLab instance, we need a set of recording rules that can be used as Service Level Indicators. Once we have the SLIs we need easy ways for administrators to view that information at a glance. A Grafana dashboard is one way to do that.