MINOR: Relax conditions to reduce rebalances with the classic consumer

[ezhou413]

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Reviewers: Lianet Magrans lmagrans@confluent.io

[lianetm]

is a boolean enough or do we need to know the nodes that are down? E.g, what about there is a rack change (need to rebalance), at the same time that a replica is down?

Seems safer to be explicit about what we will tolerate and not rebalance. We have "known replicas" (with their racks, like the line above but wonder if we need to track a Map<replicaId, rack> instead), and "unknown replicas" (replicaIds that are down, have no host/rack). Then we can only safely tolerate a rack missing in one of them but present in the other for the same replica ID (that would be exactly the condition we want and nothing more: a replica X that either has the same racks it had before, or it's down). Makes sense?

[ezhou413]

Fixed in #22768