Gather total gem memory consumption for the pre-loaded app as metrics
This is sub-issue of https://gitlab.com/gitlab-org/gitlab-ce/issues/62722
This should be very similar to what we did in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29618, but instead of gathering static data for gems, we should preload the app.
Kamil's note: "Just ensure that we effectively run in preload_app!
mode".
I prefer not to focus on parsing every single gem's data here (will do a separate follow-up issue), the main focus is to boot the production
env in CI and make derailed exec
command work. Also, here we'll use $ bundle exec derailed exec perf:mem
command, all other metrics available in the gem will be assigned to separate issues.