how to calculate burrow kafka_burrow_total_lag

Hi, Team Burrow. I'm HyoTaek

We're detecting lag through burrow.

The number in `kafka_burrow_total_lag` is too big, so I checked it,
`kafka_burrow_total_lag` was different from the sum of `kafka_burrow_partition_lag`.

Can you tell me how the `kafka_burrow_total_lag` number is determined?

thank you

No file, test, or entry point is identified. Start by locating the definitions and documentation for kafka_burrow_total_lag and kafka_burrow_partition_lag, then compare their aggregation logic. Done means documenting how total lag is calculated and explaining the reported difference.

Written by the indexing model from the issue text.