## Summary Add monitoring for total size of regular files in `pg_wal` so WAL buildup is visible in Prometheus and Grafana. ## Implementation - add a new instance-level `pg_wal_size` metric using `pg_ls_waldir()` - expose the metric in the `full` preset - surface the metric in the single-node performance overview dashboard - return a status code when the function is unavailable or lacks execute privilege ## Notes - `pg_ls_waldir()` works for users with `pg_monitor` privileges - the size covers regular files directly in `pg_wal` and excludes subdirectories such as `pg_wal/archive_status`