Current state: ["Under Discussion"]
Discussion thread: here
JIRA: here
Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).
Pre-Kraft brokers currently register 0 for every controller metric. They do this because in the case of a controller failover, the metrics of a newly elected controller will no longer be 0. KRaft does not have this issue because processes with the "broker" role are not eligible to become controllers. There is some performance overhead associated with registering metrics, so moving forward it would be best if KRaft brokers do not expose controller metrics.
The behavior of which nodes expose controller metrics will be different in KRaft clusters than the behavior when using Kafka with Zookeeper. KRaft nodes will only expose the controller metrics on the controllers. The controller metrics are calculated as metadata records are replayed e.g. OfflinePartitionCount is calculated as PartitionRecord and PartitionChangeRecord are replayed by the active controller. If the controller is a standby, it will expose 0 for the metric since it has not replayed any metadata records yet.
Pre-Kraft brokers expose 0 for controller metrics unless they are the active controller. KRaft brokers should not since they are not controller eligible.