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`.
We want our autoloading to respect custom inflections registered with ActiveSupport::Inflector. `Zeitwek::Inflector` does not call out to ActiveSupport.
Instead, we can define our own DiscourseInflector based on the super-simple Inflector in rails core.
Follow-up to 5743a6ec
