Expose replication backlog gauges (#5557)

* fix(replication): count backlog at queue admission

Co-Authored-By: heihutu <heihutu@gmail.com>

* feat(obs): expose bucket replication backlog gauges

Co-Authored-By: heihutu <heihutu@gmail.com>

* fix(obs): report recent backlog from queued work

Co-Authored-By: heihutu <heihutu@gmail.com>

* fix(obs): preserve legacy backlog metric semantics

Co-Authored-By: heihutu <heihutu@gmail.com>

* feat(obs): expose durable MRF backlog gauges

Co-Authored-By: heihutu <heihutu@gmail.com>

* test(obs): cover replication backlog metric scope

Co-Authored-By: heihutu <heihutu@gmail.com>

* fix(obs): keep backlog metrics API-compatible

Co-Authored-By: heihutu <heihutu@gmail.com>

* refactor(obs): streamline replication backlog metrics

Keep MRF backlog accounting and OBS metric collection on a single, cheaper path.

Co-Authored-By: heihutu <heihutu@gmail.com>

* test(kms): update aws capability snapshot

Co-Authored-By: heihutu <heihutu@gmail.com>

---------

Co-authored-by: heihutu <heihutu@gmail.com>

crates/kms/src/backends/snapshots/rustfs_kms__backends__aws__tests__aws_backend_capabilities.snap

@@ -10,5 +10,6 @@ expression: capabilities_snapshot(backend.capabilities())
   "physical_delete": false,
   "rotate": true,
   "schedule_deletion": true,
+  "update_key_metadata": false,
   "versioning": true
 }