KIP-847 — Add ProducerIdCount metrics

Accepted Kafka 3.5 MetricsProducer

KIP-847 adds a `ProducerIdCount` JMX metric to each broker that reports the number of active producer IDs tracked by `ProducerStateManager`. Operators had no lightweight way to monitor idempotent and transactional producer count; the only existing visibility (from KIP-360) required parsing log or snapshot data rather than querying a metric.

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