WIP: Fixes `gitaly_check`.

What does this MR do?

Based on: !17953 (merged)

Part of: #30201 (closed)

Related to: #30037 (closed)

Current payload looks like this for /-/readiness:

{"db_check":{"status":"ok"},"redis_check":{"status":"ok"},"cache_check":{"status":"ok"},"queues_check":{"status":"ok"},"shared_state_check":{"status":"ok"},"gitaly_check":{"status":"ok","labels":{"shard":"nfs-file40"}}}

After the changes it will look as:

{"db_check":{"status":"ok"},"redis_check":{"status":"ok"},"cache_check":{"status":"ok"},"queues_check":{"status":"ok"},"shared_state_check":{"status":"ok"},"gitaly_check::nfs-file01":{"status":"ok","labels":{"shard":"nfs-file01"}},...,{"gitaly_check::nfs-file40":{"status":"ok","labels":{"shard":"nfs-file40"}}}

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides