Expose lag metrics when consuming from kafka

Expose lag metrics when consuming from kafka, this could be sum across all partitions of (approx offset at broker - latest offset consumed) which would be very accurate but should be a close enough approximation.

The issue names no files or tests; start by locating Pinot's Kafka consumer and existing metrics entry points. Confirm how partition offsets are available and define done as exposing a usable lag metric based on the difference between broker and consumed offsets, aggregated across partitions.

Written by the indexing model from the issue text.