KIP-503 — Add metric for number of topics marked for deletion

Discussion Kafka 2.4 MetricsBroker

Adds broker-side JMX metrics for the number of topics currently marked for deletion and the total number of replica-deletion tasks queued in the controller. When many topics are deleted simultaneously, the controller can become overwhelmed processing deletion tasks, but operators have no metric to track how many deletions remain pending or to determine when the queue will drain.

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