After some discussion we believe there is room to improve the accuracy of this test, by enforcing the wait for a stable rebalance result after KIP-441.
System test StreamsUpgradeTest.test_metadata_upgrade could fail due to:
"Never saw output 'processed [0-9]* records' on ubuntu@worker6"
which if we take a closer look at, the rebalance happens but has no task assignment. We should fix this problem by making the rebalance result as part of the check, and wait for the finalized assignment (non-empty) before kicking off the record processing validation.