* 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.
In MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15834 we
removed use of the data produced by the Allocations Gem. However, we
never removed the code that just enables tracking of allocations. In
this commit we remove all remaining traces of this Gem.
On GitLab.com, InfluxSampler#sample_objects appears to take 1.2 s or so to
iterate through 1059 objects. This had led to delays of a couple hundred
milliseconds in processing in the main thread. Remove this code since it's not
really being used.
Closesgitlab-com/infrastructure#3250
