fix: correct index labels in multiple aggregations for DataFrameGroupBy#723

gcf-merge-on-green[bot] merged 3 commits intomaingoogleapis/python-bigquery-dataframes:mainfrom

main_chelsealin_fixgroupbymultiindexgoogleapis/python-bigquery-dataframes:main_chelsealin_fixgroupbymultiindexCopy head branch name to clipboard