KIP-177 — Consumer perf tool should count rebalance time

Accepted Kafka 1.0 ConsumerMetrics

Adds rebalance duration tracking to the `ConsumerPerformance` tool, reporting time spent in rebalance separately from message consumption time. Without this, performance benchmarks that trigger rebalances conflate consumer group coordination overhead with actual throughput, making cross-version comparisons misleading when rebalance behavior changes.

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