Add pg_wal_archive_status query for WAL monitoring

Added pg_wal_archive_status query to monitor WAL segments.

Author: shikharbhardwaj

common/assets/postgres_exporter_queries.yaml

@@ -166,3 +166,10 @@ pg_stat_database_conflicts:
     - confl_active_logicalslot:
         usage: "COUNTER"
         description: "Total queries canceled due to active logical replication slots across all databases (PG 14+)"
+
+pg_wal_archive_status:
+  query: "SELECT get_wal_ready_count() AS ready_count"
+  metrics:
+    - ready_count:
+        usage: "GAUGE"
+        description: "Number of WAL segments ready to be archived (.ready files)"