KIP-1019 — Expose method to determine Metric Measurability

Accepted Kafka 3.8 Metrics

Adds a public `isMeasurable()` method to the `KafkaMetric` interface so callers can check measurability without relying on exception handling or Java reflection. Existing code either caught `ClassCastException` from `metric.measurableValue()` or used reflection to inspect the internal value provider, both of which are fragile and expensive.

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