[{code:"X1",elements:["A","B","C","D"]},{code:"X2",elements:["C","D"]},{code:"X3",elements:["A"]}]

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