float-util-mariadb: grant replica monitor for exporter

mysqld-exporter errors out when attempting to collect slave stats:

level=error msg="Error scraping for collect.slave_status: Error 1227: Access denied; you need (at least one of) the SUPER, SLAVE MONITOR privilege(s) for this operation" source="exporter.go:171"

Option introduced in mariadb 10.5.8: https://mariadb.com/docs/reference/mdb/privileges/SLAVE_MONITOR/