Document log_processing_rules for MySQL slow query logs

[florimondmanca]

What does this PR do?

Add a missing log_processing_rule entry to the MySQL config samples.

Motivation

Without this additional setting, the upcoming MySQL slow log parser (see https://github.com/DataDog/logs-backend/pull/6293) won't be able to extract any data, as the Agent would send logs on a line-by-line basis, instead of grouped as individual entries to the slow query log.

(This also means that users were seeing slow query logs line-by-line in the logs UI up to now, which is arguably a bug since slow query log lines only make sense when viewed as a group.)

Additional Notes

/

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

[AlexandreYang]

👍