Why this change?
This is a follow up to e8f7b62752.
Tracking of GC stats didn't really belong in the `MethodProfiler` class
so we want to extract that concern into its own class.
As part of this PR, the `track_gc_stat_per_request` site setting has
also been renamed to `instrument_gc_stat_per_request`.
What does this change do?
This change adds a hidden `track_gc_stat_per_request` site setting which
when enabled will track the time spent in GC, major GC count and minor
GC count during a request.
Why is this change needed?
We have plans to tune our GC in production but without any
instrumentation, we will not be able to know if our tuning is effective
or not. This commit takes the first step at instrumenting some basic GC
stats in core during a request which can then be consumed by the discourse-prometheus plugin.
It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
