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

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