Database
[{aggDate:"2019-05-23",results:[{foo:0.58,bar:0.42},{foo:0.32,bar:0.98}]}]
Query
db.collection.aggregate([{$addFields:{avg:{$divide:[{$reduce:{input:{$objectToArray:"$result"},initialValue:0,in:{$sum:["$$value","$$this.v"]},}},{$size:{$objectToArray:"$result"}}]}}}])