Document Prometheus metrics

Problem

We were recently asked about a list of Prometheus metrics exported by the registry, and there is no such list on our docs. Metrics definitions are currently scattered across the source code in metrics.go files.

Solution

Document all exported Prometheus metrics. This can be done in https://gitlab.com/gitlab-org/gitlab/blob/master/doc/administration/monitoring/prometheus/registry_exporter.md, but we should also keep a list as a markdown doc in this repository.