Database
[{"key":1,something_special:1},{"key":2,something_related:2},{something:3},{another_one:4}]
Query
db.collection.aggregate([{$addFields:{obj_as_arr:{$objectToArray:"$$ROOT"}}},{"$addFields":{field_names:"$obj_as_arr.k",}},{"$match":{field_names:{$regex:"something"}}},{"$project":{obj_as_arr:0,field_names:0}},])