Add /-/readiness /-/liveness and /-/health_metrics endpoints to track application readiness

What does this MR do?

It adds endpoints that allow monitoring of specific GitLab dependencies.

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #24240 (closed)

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• 382d93b7 - WIP

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• 382d93b7...b57ed57f - 236 commits from branch gitlab-org:master
• 7c3fd382 - wip
• 93efe95d - evolution of WIP
• 26e1a508 - clean up unecessary code
• d4d6578f - reform fs tests
• cc54bc54 - that code was really useless
• 81890d41 - DB and Redis check tests
• e629c0c1 - SimpleAbstract check
• 76d588b5 - Redis use abstract check
• bdb5477c - Refactor
• 70939513 - Cleanup tests
• acb6b367 - Health Controller
• 610a5546 - rename health::checks to health_checks
• d63de2e6 - add labels to HC result
• f67472ce - Working metrics endpoint
• e030cf05 - WIP

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• 1a7cdb1f - Add health controller tests

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• faadcadb - Add /-/readiness /-/liveness and /-/health_metrics endpoint

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• ad8cf3c1 - Fix rubocop warnings

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• 05aee836 - Check also fallback methods

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• d9d1dc8a - Enable timeouts

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• 6aa4866b - Rubocop fixes

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unmarked as a Work In Progress

changed title from WIP Add /health endpoint to track application readiness to Add /-/readiness /-/liveness and /-/health_metrics endpoints to track application readiness

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• 404e69e5 - Add Changelog

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• c22c1fd5 - Fix rubocop and MySql compatibility

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• c22c1fd5 - Fix rubocop and MySql compatibility

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• cabf33cb - Fix rubocop and MySql compatibility

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marked the task Changelog entry added, if necessary as completed

marked the task Documentation created/updated as completed

marked the task Added for this feature/bug as completed

marked the task All builds are passing as completed

marked the task API support added as completed

marked the task Added for this feature/bug as incomplete