Fix only_custom_queries configuration not honored

[alexandre-normand]

What does this PR do?

This fixes support for the only_custom_queries. Prior to this change, all core mysql metrics were still emitted even with only_custom_queries set to true.

Motivation

A customer report.

Additional Notes

performance_schema is being used by the statement metrics job loop when dbm is enabled. Previously, this was gathered with other global status variables in the main metric collection but this had to be extracted now to always be initialized regardless of whether the core integration metrics were collected or not.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #11506 (742af40) into master (e89bb02) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Flag	Coverage Δ
mysql	87.75% <100.00%> (+0.37%)	⬆️

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[alexbarksdale]

LGTM