Issue description: I just ran into `/var/opt/gitlab` on a (small, home use) Omnibus install consuming way more space than anticipated. Turned out `/var/opt/gitlab/prometheus` consumed just shy of 6 Gigs, while the rest of `/var/opt/gitlab/` is just shy of 2 Gigs. Monitoring data consuming thrice the space needed for my repos and issues came as a surprise. Since I also never once looked at the built-in prometheus, I followed your docs at https://docs.gitlab.com/13.3/ee/administration/monitoring/prometheus/index.html#configuring-prometheus to disable it. So far so good, the docs were easy to locate and the steps clear, BUT; After a `gitlab-ctl reconfigure`, `/var/opt/gitlab/prometheus` was still present (that's OK), but it was not clear from the 3 steps under "To disable Prometheus and all of its exporters, as well as any added in the future:" what can and should be deleted on the filesystem after setting `prometheus_monitoring['enable'] = false`. Request: Can you please add to that section of the docs which directories can be safely deleted after disabling prometheus? Additional info: Since it gets enabled by default since 9.0, I'll presume I'm not the only one with a small EE instance that will want to disable prometheus for space saving reasons. FWIW: I've deleted that one large directory with seemingly no ill effect, but clarification in the docs plus the info if there are other places in the filesystem I can clean would be appreciated. <!-- Don't edit below this line -->