MongoDB Aggregation – Flatten resultat

db.collection.aggregate([
              ....
    { $group: {_id: "$data.r.id", "col": {$first: "$data.r"}}},
    {$project:{"o":{"$objectToArray":"$col"}}},
    {$unwind:"$o"},
    {$group:{"_id":null, "keys":{$addToSet:"$o.k"}}},
    {$project: {"keys":1, "_id": 0}},
    {$addFields: {res:{$map:{ input:"$keys", as: "a", in: {"_id":0,"label": "$$a"}}}}},
    {$project: {"res":1}}
])

With my MongoDB query, the output is:

{ 
  "res" : [
     {
         "_id" : 0.0, 
         "label" : "aaaa"
     }, 
     {
         "_id" : 0.0, 
         "label" : "bbbbb"
     }, 
     {
         "_id" : 0.0, 
         "label" : "ccccc"
     }
  ] 
}

and I try to output this:

{
    "_id" : 0.0, 
    "label" : "aaaa"
}, 
{
    "_id" : 0.0, 
    "label" : "bbbb"
}, 
{
    "_id" : 0.0, 
    "label" : "cccc"
}

so I added:

{$replaceRoot : {"newRoot": "$res"}}

but I got this error:

”newRoot’ expression must evaluate to an object, but resulting value
was: [{_id: 0, label: "aaaa"},{_id: 0, label: "bbbb"},{_id: 0, label:
"cccc"}]

How can I remove the nested object "res" to return only the list/array of objects (id, label)?