KIP-1068 — New metrics for the new KafkaConsumer

Accepted Kafka 4.0 ConsumerMetrics

Adds new metrics specific to the `AsyncKafkaConsumer` (CONSUMER rebalance protocol) including thread coordination metrics and event queue depth that are not applicable to the `LegacyKafkaConsumer` (CLASSIC protocol). The new consumer implementation introduced in KIP-848 uses a dual-thread model (API thread + network I/O background thread) whose operational characteristics cannot be observed with the existing single-threaded consumer metrics.

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