Add Burrow for consumer lag monitoring

[solsson]

It's a quite useful REST API, but I will try to add Prometheus scrape support (pending linkedin/Burrow#318).

[solsson]

There's no service now, so to test run kubectl port-forward 8000:8000 and curl for example http://localhost:8000/v3/kafka/local/consumer
Use 8080:8080 for metrics, with http://localhost:8080/metrics

[solsson]

We've found Burrow truly useful. It's lightweight, fast and in addition to crucial metrics it provides quick access to for example topic listings, e.g. (from inside your cluster) curl -s http://burrow.kafka/v3/kafka/local/topic | jq -r ".topics|.[]" and consumer listings curl -s http://burrow.kafka:8000/v3/kafka/local/consumer | jq -r ".consumers|.[]".

With Prometheus, this PRs title is realized with the metric kafka_burrow_partition_lag and its labels topic and group.