fix pg_replication_lag_seconds (#895)

Signed-off-by: Vladimir Luksha <[email protected]>
Co-authored-by: Vladimir Luksha <[email protected]>

Author: IamLuksha

collector/pg_replication.go

@@ -55,6 +55,7 @@ var (
 	pgReplicationQuery = `SELECT
 	CASE
 		WHEN NOT pg_is_in_recovery() THEN 0
+                WHEN pg_last_wal_receive_lsn () = pg_last_wal_replay_lsn () THEN 0
 		ELSE GREATEST (0, EXTRACT(EPOCH FROM (now() - pg_last_xact_replay_timestamp())))
 	END AS lag,
 	CASE