Update the list of GitLab metrics to include the missing metric names.

Now the files are identical again compared to EE.

These are backported from
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5050

Was introduced in the time that GitLab still used NFS, which is not
required anymore in most cases. By removing this, the API it calls will
return empty responses. This interface has to be removed in the next
major release, expected to be 12.0.

* Use a simple counter for sampler duration instead of a histogram.
* Use a counter to collect GC time.
* Remove unused objects metric.
* Cleanup metric names to match Prometheus conventions.
* Prefix generic GC stats with `gc_stat`.
* Include worker label on memory and file descriptor metrics.

 - Change the wording a little bit

Document the original release of each Prometheus metric.

* Use `_total` naming convention.
* Add to metrics documentation.

Add `seconds` unit to latency metrics based on uptream naming
convention[0].

[0]: https://prometheus.io/docs/practices/naming/#metric-names