KIP-272 — Add API version tag to broker's RequestsPerSec metric

Accepted Kafka 2.0 MetricsProtocol

Adds an API version tag to the broker's `RequestsPerSec` JMX metric, breaking it down into per-API-version counters (e.g., `ProduceRequest.v5`, `FetchRequest.v9`). During rolling upgrades, operators cannot determine what fraction of clients have upgraded to a new API version, making it unsafe to change `message.format.version` or deprecate old request versions.

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