CONSUMER LAG = LOG_END_OFFSET - CURRENT-OFFSET. The lag is to be computed per conusmer group (LEFT JOIN on topic, partition, consumer group). As each partition acts as a queue itself, the lag is to be computed per partition. Having lag per partition, one can compute total consumer lag and max consumer lag + compare lag for being hopefully about evenly distributed between partitions for one conusmer group.
In theory the consumergroup metricset can be used to get the missing stats in metricbeat. But unfortunately this feature (still in beta btw.) contains a bug, not properly collecting the data.