The onRetry hook in TestOffsetManagement/ConsumerGroupResumption only fires
after defer reader.Close(), so every dump shows the post-LeaveGroup Empty
state — useless for diagnosing why the second consumer hangs in the join
cycle. Add an onTick callback fired every 1.5s while the reader is still
joined so we can see PreparingRebalance / CompletingRebalance churn,
leader, and member assignments during the 20s attempt window.
6aa353716a