request

db.getCollection('tost').aggregate(
[
    {
        "$group":{
            "_id": "$_id",
            "doc_info":{
                "$first": "$doc_info"
            },
            "wa_count": {
                "$first": {
                    "$size": "$wa"
                }
            },
            "ax_count": {
                "$first": {
                    "$size": "$wa.ax"
                }
            }
        }
    }
])