add metrics for partitions

[jsurloppe]

Hi,

This PR add 3 metrics:

• kminion_kafka_topic_partition_count: give the partition_count as value for being able to used in prom queries.
• kminion_kafka_topic_under_replicated_partition: set to 1 if a partition is currently under replicated.
• kminion_kafka_topic_partition_leader: current leader (broker ID)

That's 3 metrics that we miss from kminion, it would be nice to have them in all in one place. ;)

[CLAassistant]

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