`pg_stat_replication_pg_wal_lsn_diff` metric needs superuser

postgres_exporter deprecated pg_stat_replication_pg_xlog_location_diff in favour of pg_stat_replication_pg_wal_lsn_diff starting from postgres 10. The latter metric needs superuser permissions, see https://github.com/wrouesnel/postgres_exporter/issues/97.

We should either promote the gitlab_exporter postgres user to superuser or create corresponding functions with security permissions.