Instrument create_time from last message in batch

[jonmagic]

Why?

My team at GitHub uses racecar version 1.0.1 and by extension ruby-kafka. We're interested in the potential of racecar 2 that uses the rdkafka-ruby client but we heavily rely on the consumer.offset_lag metric for monitoring our stream processors.

It doesn't look like that metric is going to be straightforward to implement in racecar 2 but I'm wondering if we could instead switch to the consumer.time_lag metric instead? Unfortunately that metric isn't published in the process_batch.consumer payload in racecar 1.0.1 or racecar 2.0.0-beta.

How?

This PR updates the process_batch.consumer payload to include a last_create_time metric which comes from the last message in the batch.

We could use a fork of ruby-kafka with these changes to accomplish our goal with racecar 1.0.1 in the short term but I thought others may benefit as well so I'm opening this PR.

I'm also hoping to open a PR against racecar 2 to add the same metric to the process_batch payload so we can easily transition from racecar 1.0.1 with ruby-kafka to racecar 2.0.0 with rdkafka-ruby.

Thank you for your consideration.

[jonmagic]

The failing specs should all pass once #810 ships and I update this PR to include it.

[dasch]

Can you rebase on master?

[dasch]

Thanks!