Document Prometheus metrics for workhorse

Problem to solve

The metrics for workhorse are absent from: https://docs.gitlab.com/ee/administration/monitoring/prometheus/gitlab_metrics.html

Further details

We can see the metrics definitions using this search: https://gitlab.com/search?search=promauto.New&nav_source=navbar&project_id=278964&group_id=9970&search_code=true&repository_ref=master

Proposal

Add a 'Workhorse metrics' section to https://docs.gitlab.com/ee/administration/monitoring/prometheus/gitlab_metrics.html and document all metrics.

Who can address the issue

Workhorse expertise

Other links/references