pg_replication_slots in standby (#147)

WAL functions cannot be executed in standby servers
```
ERROR:  recovery is in progress
HINT:  WAL control functions cannot be executed during recovery.
```

In PostgreSQL 16 replication slots persist in the standby servers.
This is also the case when using extensions like pg_failover_slots that transfer the slot information to the standby.

This condition will make those queries to return values only in the primary node avoiding the errors.

Co-authored-by: Jiří Čtvrtka <[email protected]>
Co-authored-by: Nurlan Moldomurov <[email protected]>

cmd/postgres_exporter/queries.go

@@ -95,13 +95,15 @@ var queryOverrides = map[string][]OverrideQuery{
 			`
 			SELECT slot_name, database, active, pg_xlog_location_diff(pg_current_xlog_location(), restart_lsn)
 			FROM pg_replication_slots
+                        WHERE pg_is_in_recovery() = False
 			`,
 		},
 		{
 			semver.MustParseRange(">=10.0.0"),
 			`
 			SELECT slot_name, database, active, pg_wal_lsn_diff(pg_current_wal_lsn(), restart_lsn)
 			FROM pg_replication_slots
+                        WHERE pg_is_in_recovery() = False
 			`,
 		},
 	},