Hello, Kafka team. When running in production the common user question is «How big lag between producer and consumer?». We have a `kafka-consumer-groups.sh` tool and `AdminClient#getListConsumerGroupOffsetsCall` to answer the question.
Even detailed guides on how to calculate *consumer lag* with built-in Kafka tools, exists. [1] Obviously, approach with tool or AdminClient requires additional coding and setup which can be inconvenient. I think Kafka should provide per partition consumer offset metric. It will simplify running Kafka deployment and monitoring in production. Looked in `GroupMetadataManager.scala` and think it possible to add those metrics. What do you think? Do we need this metrics on Kafka broker? [1] https://www.baeldung.com/java-kafka-consumer-lag
