KAFKA-6588: Add metric for alive log cleaner thread count

[navina]

This patch introduces a Log Cleaner metric - live-cleaner-thread-count to monitor the log cleaner thread. It additionally fixes a minor issue to ensure that the correct offsets are logged in LogCleaner#recordStats.

[navina]

@becketqin review please. Thanks!

[ijuma]

Thanks for the PR. New metrics require a KIP: https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Improvement+Proposals

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