Every time we create a new toolsdb instance, a new `gtid_domain_id` is [generated](https://gerrit.wikimedia.org/r/plugins/gitiles/operations/puppet/+/refs/heads/production/modules/mariadb/manifests/config.pp#48) and written to `/etc/my.cnf`.
This should not be necessary as we're not using multi-source replication, and it only ends up messing up the GTID values if a replica is promoted to primary and both the old and new `gtid_domain_id` become part of `gtid_binlog_pos`:
```
gtid_binlog_pos | 2886731301-2886731301-4714442229,2886731673-2886731673-4887243158
```
`gtid_domain_id` was recently set to `0` in some production servers ({T336228}) but that task was //Declined// in favor of a [different approach](https://phabricator.wikimedia.org/T324965#9125619).
ToolsDB is slightly easier because we only have one primary and one replica. I think we could create a new replica host (e.g. `tools-db-3`) where we set `gtid_domain_id = 0`, then failover the existing primary to that new replica. New transactions after the failover will use the new domain id, and we can finally clean up the old domain ids, as we did for older domain ids in {T334947}.
I think it would also be nice to set `server_id` to the last token of the hostname (e.g. `3` for `tools-db-3`) instead of [deriving it](https://gerrit.wikimedia.org/r/plugins/gitiles/operations/puppet/+/refs/heads/production/modules/mariadb/manifests/config.pp#43) from the hostname. This means we would get more readable GTIDs like `0-3-4714442229` instead of `2886731301-2886731301-4714442229`.
Finally, we should set [gtid_strict_mode](https://mariadb.com/kb/en/gtid/#gtid_strict_mode) to 1, as it can prevent some GTID-related issues.
Cc-ing @Marostegui for review.