Database
[{_id:1,"tag":{"tagName":0.10534846782684326}},{_id:2,"tag":{".tagName":0.10534846782684326}}]
Query
db.collection.aggregate([{"$addFields":{"tagNames":{"$objectToArray":"$tag"}}},{$match:{"tagNames.k":{$in:["tagName",".tagName"]}}},{$project:{tagNames:0}}])