feat: add replication info to /metrics endpoint

[EricHayter]

Exposes 4 new fields to the /metrics endpoint relating to replication info:

• master_link_status
• master_last_io_seconds_ago
• master_sync_in_progress
• slave_repl_offset

This commit also adds integration testing to validate the new functionality.

Fixes #6183

[augmentcode]

🤖 Augment PR Summary

Summary: This PR exposes replica-side replication state in the Prometheus /metrics endpoint.

Changes:

• Adds four new metrics derived from ReplicaSummary: master link status, seconds since last master IO, full-sync-in-progress flag, and replica offset.
• Emits these metrics only when the server is operating in replica mode (m.replica_side_info is present).
• Exports slave_repl_offset only after a full sync has completed (or after a completed full sync when the link is currently down), matching existing INFO behavior.
• Adds an integration test that verifies the new metrics appear on a replica and do not appear on a master.

Technical Notes: The implementation reuses the existing replication summary fields already used by the INFO replication output, keeping metrics consistent with runtime replication state.

_{🤖 Was this summary useful? React with 👍 or 👎}

[augmentcode]

Review completed. 2 suggestions posted.

Comment augment review to trigger a new review at any time.

[romange]

@EricHayter thanks for working on it.

[romange]

Please sign your commits @EricHayter

[EricHayter]

Please sign your commits @EricHayter

I believe my commits already include a sign-off, but the PR description does not. Should I also add the sign-off to the PR message?

[romange]

I meant with sign with gpg key, see #7122 for details.

[EricHayter]

Sorry for the delay on getting this signed. I initially amended the commit locally, but didn’t realize I also needed to add my public key to GitHub for the signature to be recognized. It should be properly signed now.

[romange]

Thank you, Eric!