KIP-168 — Add GlobalTopicCount and GlobalPartitionCount metric per cluster

Accepted Kafka 1.0 MetricsBroker

Adds `GlobalTopicCount` and `GlobalPartitionCount` gauge metrics on the Kafka Controller, reflecting the total number of topics and partitions across the entire cluster. No existing metric gives a cluster-wide topic/partition count; operators must call `listTopics()` or query ZooKeeper to answer this basic capacity question.

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