Function chunk_spans_across_groups 

fn chunk_spans_across_groups(
    groups: Vec<TraceSpanGroup>,
    chunk_size: usize,
) -> Vec<Vec<TraceSpan>>

Chunks the spans of all resource/scope groups together. Every span carries its own resource and scope attributes, so a chunk may mix groups; chunking per group instead would turn a collector batch of many small groups into many tiny inserts, each fanned out to every trace partition.