[{type:"A",completed_at:"12-10-2021",owner:"xyz"},{type:"A",completed_at:"12-10-2021",owner:"abc"},{type:"C",completed_at:"12-10-2021",owner:"xyz"},{type:"C",completed_at:"13-10-2021",owner:"xyz"},{type:"B",completed_at:"13-10-2021",owner:"xyz"}]

db.collection.aggregate([{$group:{_id:{owner:"$owner",completed_at:"$completed_at"},type:{$push:"$type"}}},{$group:{_id:"$_id.owner",completed_at:{$push:"$$ROOT"}}},{$set:{completed_at:{$map:{input:"$completed_at",in:{completed_at:"$$this._id.completed_at",A:{$size:{$filter:{input:"$$this.type",as:"t",cond:{$eq:["$$t","A"]}}}},B:{$size:{$filter:{input:"$$this.type",as:"t",cond:{$eq:["$$t","B"]}}}},C:{$size:{$filter:{input:"$$this.type",as:"t",cond:{$eq:["$$t","B"]}}}}}}}}}])