Kafka metrics

[Naireen]

Add per worker metric for Kafka poll latency. This is specifically for Dataflow V1 runner.

Next steps would be to extend this to add latency for other RPC calls.

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[Naireen]

R: @scwhittle

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[scwhittle]

Can you clarify in PR description that this is for Dataflow V1 runner?

Adding John as he is more familiar with BeamIO in general and may have suggestions on monitoring.

[Naireen]

Run Java PreCommit

[sjvanrossum]

@Naireen @johnjcasey The changes introduced in this PR assume that a single KafkaUnboundedSource will be assigned a single Kafka topic. As far as I'm aware that is not what the transform allows users to configure, withTopics(), withTopicPartitions() and withTopicPattern() permit multiple topics. If the number of splits is less than the number of topic partitions, then a single split may end up with an assignment consisting of partitions from multiple topics.