[{"readings":[{"t":"temperature","r":6},{"t":"humidity","r":66}],"created_at":ISODate("2021-02-24T09:45:09.858Z"),"updated_at":ISODate("2021-02-24T09:45:09.858Z")},{"readings":[{"t":"temperature","r":5.6},{"t":"humidity","r":66}],"created_at":ISODate("2021-02-24T09:45:09.858Z"),"updated_at":ISODate("2021-02-24T09:45:09.858Z")}]

db.collection.aggregate([{$match:{"readings.t":"temperature"}},{$unwind:"$readings"},{$match:{"readings.t":"temperature"}},{$project:{readings:["$readings.r",{$toLong:"$created_at"}]}},{$group:{_id:null,readings:{$push:"$readings"}}}])