postgresql: Collect Wal-G metrics with statsd_exporter

_wal-g_ can send StatsD metrics when it completes an upload/backup/etc.
task.  Using the `statsd_exporter`, we can capture these metrics and
make them available to Victoria Metrics.

group_vars/postgresql.yml

@@ -59,3 +59,4 @@ wal_g_pg_config:
   WALG_S3_PREFIX: s3://pgbackup/pyrocufflink/main/15
   AWS_ENDPOINT: https://s3.backups.pyrocufflink.blue
   PGHOST: /run/postgresql
+  WALG_STATSD_ADDRESS: localhost:9125