feat(localdb): telemetry meters + ISyncStatus

Claude-Session: https://claude.ai/code/session_01BL2Vu1ESDQ9SCN4gVKkdts

ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/Internal/OplogStore.cs

@@ -79,6 +79,21 @@ internal sealed class OplogStore(ILocalDb db, ReplicationOptions options, Func<D
         return rows[0];
     }
 
+    /// <summary>
+    /// The unacked backlog read synchronously on a dedicated short-lived connection, for the
+    /// metric-observation callbacks (gauge scrape / status poll). Cheap enough at that frequency;
+    /// mirrors <see cref="GetOplogDepthAsync"/>'s query. May throw if the db is disposing — callers
+    /// on the observation path swallow and drop the sample.
+    /// </summary>
+    public long GetOplogDepthSync()
+    {
+        using var conn = db.CreateConnection();
+        using var cmd = conn.CreateCommand();
+        cmd.CommandText =
+            "SELECT COUNT(*) FROM __localdb_oplog WHERE seq > (SELECT last_acked_seq FROM __localdb_peer_state WHERE id = 1)";
+        return (long)cmd.ExecuteScalar()!;
+    }
+
     /// <summary>The highest oplog seq present, or 0 when the oplog is empty.</summary>
     public async Task<long> GetMaxSeqAsync(CancellationToken ct = default)
     {