[{_id:1,"tag":{"tagName":0.10534846782684326}},{_id:2,"tag":{".tagName":0.10534846782684326}}]

db.collection.find({$expr:{$let:{vars:{tagName:{"$arrayElemAt":[{"$objectToArray":"$tag"},0]}},in:{$in:["$$tagName.k",["tagName",".tagName"]]}}}})