Feature Request: add kafka_consumergroup_lag_seconds metric

[theonlyguyy]

As suggested in issue - Estimated lag duration #229, exporting a metric that indicates how long it would take the consumer/comsumer_group to catch up to the current produced offset in the topic/partition.

Since the exporter already collects and exports metrics on consumers, consumer-groups, offset, lag and partitions,
is it a possibility to add it as an exposed metric?

This metric could help are clients to better and more intuitively understand their consumer pace.
Thanks in advance.