Function build_series_index 

pub(crate) async fn build_series_index(
    store: &ObjectStore,
    region: &MitoRegionRef,
    version: &Arc<Version>,
    bucket: &SeriesBucket,
    entry: &SeriesIndexEntry,
    purger: &IndexFilePurger,
) -> Result<SeriesIndexFileHandle>

Builds only the series index. Callers build needed range indexes separately.