KIP-843 — Adding addMetricIfAbsent method to Metrics

Accepted Kafka 3.3 Metrics

Adds `Metrics.addMetricIfAbsent(MetricName, Measurable)` as an atomic get-or-create operation on the metrics registry. The existing two-step pattern (check existence, then `addMetric`) is not thread-safe: concurrent threads registering the same instance-level metric race and one throws `IllegalArgumentException`.

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