The current status:

• the following instances have extra-port set:
  • core single-instance hosts (via modules/role/templates/mariadb/mysqld_config/production.my.cnf.erb): 3307
  • m3 multi-instance hosts (via modules/profile/templates/mariadb/mysqld_config/phabricator_instance.my.cnf.erb): 3343
• profile::mariadb::ferm assumes the extra-port is port+20 for multi-instances.

In T257033 we standardized the mapping of section name to mariadb port for multiinstance hosts.
In https://gerrit.wikimedia.org/r/c/operations/puppet/+/644453 we added a new mapping entry for x2 at 3330.

From conversation with @Marostegui, the core single-instance hosts are the most important ones to have, which are already covered.

It would be good to have a standard mapping for extra-port's so we can make sure we don't end up with conflicts.