Account for logical replication slots on replicas with patroni

[keithf4]

Describe the issue: postgres_exporter gives scrape error warnings on replicas when permanent logical replication slots are in use with patroni.

postgres=# SELECT slot_name, active::int, pg_wal_lsn_diff(pg_current_wal_insert_lsn(), restart_lsn) AS retained_bytes FROM pg_catalog.pg_replication_slots;
ERROR:  recovery is in progress
HINT:  WAL control functions cannot be executed during recovery.

Describe the expected behavior: Normally there's no slots on a replica, so this would return null there. However, patroni makes "permanent" replication slots that exist on replicas. Either have to remove the retained_bytes data or rewrite the query to work despite it.

Tell us about your environment:

• pgMonitor version: 4.6
• Container or non-container:
• Container name / image:
• Operating System for non-container: R7
• PostgreSQL Version: 13
• Exporter(s) in use (incl. version): postgres_exporter 0.10.1
• Prometheus version:
• AlertManager version:
• Grafana version: