[{color:"red",value:{red:{level1:{level2:5}}}},{color:"blue",value:{blue:{level1:{level2:8}}}}]

db.collection.aggregate([{"$bucket":{groupBy:{$let:{vars:{value:{$objectToArray:"$value"}},in:{$arrayElemAt:["$$value.v.level1.level2",0]}}},boundaries:[1,2,3,4,5,6,7,8,9],output:{count:{"$sum":1}}}}])