Expand the instrumentation FF coverage [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

I noticed that we need to add more code under the method_instrumentation_disable_initialization feature flag as it exclusively serves for the instrumentation init statements.

We could've probably jumped over this step and remove this code alongside the FF removal, but I prefer to move in tiny steps here because it's an initializer.
Even though the code shouldn't do anything outside the Instrumentation, I have a tiny concern about the possible implicit effects of no longer calling the require_dependency file.

The diff may look messy, it's worth checking the whole file: https://gitlab.com/gitlab-org/gitlab/-/blob/257d5ebd1636a8d35c130f782d148c1c5c04d3ff/config/initializers/zz_metrics.rb#L167

🗒 ⛳ To do the verification properly, I disabled the FF on production again: https://gitlab.slack.com/archives/C101F3796/p1631013557358300 . I will re-enable and monitor it after this MR is released.

How to setup and validate locally (strongly suggested)

Make sure that the application restarts with and enabled/disabled FF:

rails c >
::Feature.disable(:method_instrumentation_disable_initialization)
bash > 
gdk restart rails-web

and

rails c >
::Feature.enable(:method_instrumentation_disable_initialization)
bash > 
gdk restart rails-web

Check for logs and that the app started fine.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

N/A

Related to #217978 (closed)