Feature Description

This issue requests the ability for a Vitess user to override the Durability Policy promotion rule of an individual tablet

This feature comes with some risks (if must_not is used too widely) and should be avoided unless there is a good reason, however it is a powerful functionality to have in advanced deployments

Use Case(s)

A Vitess user that would like to override the promotion rule of certain tablets, example: an experiment or upgrade where a promotion to PRIMARY is not ideal: a MySQL/OS upgrade, soak of experimental hardware/tunings, a cell/datacenter where PRIMARY is not (yet) preferred, etc

A direct example: during Slack's rollout of MySQL 8.x, it was critical that we didn't let an unsoaked/unvalidated 8.x tablet become PRIMARY. With old-Orchestrator this was possible with the flexible promotion rule support, but in a VTOrc world this is less straightforward

Also the opposite case is possible, in some cases a user may prefer an individual tablet wins a promotion (for whatever reason)