Function group_offsets 

pub fn group_offsets(
    group: &str,
    fetched: &BTreeMap<(String, i32), i64>,
) -> GroupOffsets

Turn one OffsetFetch answer into the captured shape.

The answer arrives as a map, so the ordering is already (topic, partition) and the captured file is byte-stable across two captures of an unchanged cluster. That is what lets an operator diff two captures.