KIP-703 — Add a metric for reporting idle connections closed

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Introduces a new JMX metric `connection-close-total` (tagged by `close-reason=idle`) on the `Selector` to count the number of connections closed due to exceeding `connection.max.idle.ms`. Currently, idle connection closures are only visible at TRACE log level, making it impossible to monitor this behavior from standard metrics/alerting infrastructure.

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