Bas Harenslak explains how to monitor consumer lag in Kafka:
So you’ve written e.g. a Spark ETL pipeline reading from a Kafka topic. There are several options for storing the topic offsets to keep track of which offset was last read. One of them is storing the offsets in Kafka itself, which will be stored in an internal topic
__consumer_offsets. If you’re using the Kafka Consumer API (introduced in Kafka 0.9), your consumer will be managed in a consumer group, and you will be able to read the offsets with a Bash utility script supplied with the Kafka binaries.
The Prometheus mentioned in the article is an open-source monitoring solution.