Database
[{"_id":"0001","n1":"0001","n2":1234},{"_id":"0002","n1":"0002","n2":9876}]
Query
db.collection.aggregate([{$project:{n:{n1:"$n1",n2:"$n2"}}},{$project:{lookupFields:{$objectToArray:"$n"}}},{$addFields:{lookupFields:{$map:{input:"$lookupFields",as:"lf",in:{n:"$$lf.k",v:"$$lf.v"}}}}}])