Add JSON logs support for merge services namespace

What does this MR do?

It prepares MergeRequests services while also adding an error log to MergeabilityCheckService.

TODOs

• Backport to EE (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14893)
• Update docs

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content