Database
[{"ts":"2021-09-04T12:18:41Z","te":"2021-09-04T14:59:50Z","td":"2021-09-04","ti":2.69},{"ts":"2021-09-04T15:00:00Z","te":"2021-09-04T15:05:00Z","td":"2021-09-04","ti":0.08},{"ts":"2021-09-04T15:40:00Z","te":"2021-09-04T16:00:00Z","td":"2021-09-04","ti":0.33}]
Query
db.collection.aggregate([{$group:{_id:null,docs:{$push:"$$ROOT"},ts:{$first:"$ts"},te:{$last:"$te"},ti:{$sum:"$ti"}}},{$project:{docs:{$concatArrays:[[{ts:"$ts",te:"$te",ti:"$ti"}],"$docs"]}}},{$unwind:"$docs"},])