how to get data mongo data using aggregate mongodb

i am trying to use aggregate till now i am successfull to get the data but i got a issue that how i can get the data unread. this is my mongo db, i want to extract data according to my requirement

{
    "_id": ObjectId("5912c7240520df77f0c2c18a"),
    "email_id": "54",
    "unread": "false",
    "__v": NumberLong(0),
    "tag_id": ["0"

    ]
}, {
    "_id": ObjectId("5912c71e0520df77f0c2c189"),
    "email_id": "55",
    "unread": "false",
    "__v": NumberLong(0),
    "tag_id": [
        "1"
    ]
}, {
    "_id": ObjectId("5912c71d0520df77f0c2c186"),
    "email_id": "51",
    "unread": "false",
    "__v": NumberLong(0),
    "tag_id": [
        "2", "1"
    ]
}

expected result:

{
    "data": [{
        "tag_id": "1",
        "count_email": 1,
        "unread": 9
    }, {
        "tag_id": "3",
        "count_email": 45,
        "unread": 3
    }, {
        "tag_id": "2",
        "count_email": 5,
        "unread": 4
    }, {
        "id": null,
        "count_email": 52,
        "unread": 35
    }]
}

via Shekhar Tyagi