[{item:"a",payload:{"url_google.com":1,"url_apple.com":1}},{item:"b",payload:{"url_t.co":1,"t.co":1}},{item:"c",payload:{"google.com":2,"facebook.com":2}}]

db.collection.aggregate([{$addFields:{payload_array:{$objectToArray:"$payload"}}},{$match:{"payload_array.k":{$regex:"^url_"}}},{$project:{payload_array:0}}])