MongoDB Aggregation - Output Non-existent Dates

I have a collection of ProductViews.

ProductView

{
    productId: '5b8c0f3204a10228b00a1745,
    createdAt: '2018-09-07T17:18:40.759Z',
}

And I have a query for fetching the daily views for a specific product.

Query

ProductView.aggregate([
    { $match: { productId } },
    { $project: { day: { $substr: ["$createdAt", 0, 10] } } },
    {
        $group: {
            _id: "$day",
            count: { $sum: 1 },
            time: { $avg: "$createdAt" },
        }
    },
    { $sort: { _id: 1 } },
    {
        $project: {
            date: '$_id',
            views: '$count',
        },
    },
]).exec((err, result) => ...)

Current Results

[
    { date: '2018-09-01', views: 1 },
    { date: '2018-09-02', views: 3 },
    { date: '2018-09-04', views: 2 },
    { date: '2018-09-05', views: 5 },
    // ...
]

Issue

The issue is, that this aggregation does not return { date: '2018-09-03', views: 0 } for days with 0 views. This results in incorrect displaying of the data:

Results should look like

[
    { date: '2018-09-01', views: 1 },
    { date: '2018-09-02', views: 3 },
    { date: '2018-09-03', views: 0 },
    { date: '2018-09-04', views: 2 },
    { date: '2018-09-05', views: 5 },
    // ...
]

from MongoDB Aggregation - Output Non-existent Dates