[{field1:"foo",field2:{12322:{"some":"a"},244:{"some":"b"},8791:{"some":"c"}}},{field1:"bar",field2:{789:{"some":"a"},244:{"some":"b"},8791:{"some":"c"},12:{"some":"d"}}}]

db.collection.aggregate([{$project:{data:{"$objectToArray":"$field2"}}},{"$unwind":"$data"},{"$group":{"_id":"$data.k","count":{"$sum":1}}},{"$group":{"_id":null,"data":{"$push":{k:"$_id",v:"$count"}}}},{$project:{data:{"$arrayToObject":"$data"}}},{"$replaceRoot":{"newRoot":"$data"}}])