[{"_id":{"year":2020,"month":11,"raceEthnicity":"Asian"},"count":1.0},{"_id":{"year":2020,"month":11,"raceEthnicity":"Multiracial"},"count":3.0},{"_id":{"year":2020,"month":11,"raceEthnicity":"White"},"count":3.0},{"_id":{"year":2020,"month":10,"raceEthnicity":"White"},"count":33.0}]

db.collection.aggregate([{$replaceRoot:{newRoot:{"$mergeObjects":["$$ROOT",{$arrayToObject:[[{k:"$_id.raceEthnicity",v:"$count"}]]}]}}},{"$group":{"_id":{year:"$_id.year",month:"$_id.month",},"Asian":{"$sum":"$Asian"},"Multiracial":{"$sum":"$Multiracial"},"White":{"$sum":"$White"}}}])