[{_id:ObjectId("5d8cac657d2d1e0145268fb4"),values:["5d8a2c3d4768d9660d3ba383","5d8a1d4f4768d951cb6c8989"]},{_id:ObjectId("5d8caf4039321e7b14061a46"),values:["5d8a2c3d4768d9660d3ba383","5d8c835a39321e7b15792353"]}]

db.collection.aggregate([{"$unwind":"$values"},{"$group":{"_id":"$values","count":{"$sum":1}}},{"$group":{"_id":null,"values":{"$push":"$$ROOT"}}},{"$project":{"_id":0}}])