Add feature flag to enforce gitaly request limits

What does this MR do?

Adds a feature flag to allow us to enable the Gitaly N+1 detector in our test environments. See https://gitlab.com/gitlab-org/gitlab-ce/issues/57911

Environments like staging and our upcoming performance test environment are production-like, so Rails.env.production? will return true, and so without this change the Gitaly N+1 detector would be disabled.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/57911

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer