如何使用聚合来统计文档并汇总特定字段?

我试图统计与列表ID匹配的潜在客户数量,并将其与成本相结合。这是我的收藏:

[
  {
    "list_id": "5f76a53e8a93be772c14cc06",
    "fname": "John",
    "lname": "Doe",
    "cost": 0
  },
  {
    "list_id": "5f76a53e8a93be772c14cc06",
    "fname": "John",
    "lname": "Wayne",
    "cost": 0.65
  },
  {
    "list_id": "5f76a53e8a93be772c14cc06",
    "fname": "John",
    "lname": "Kelly",
    "cost": 1
  },
  {
    "list_id": "5f76a53e8a93be772c14cc06",
    "fname": "John",
    "lname": "Morrie",
    "cost": 0.40
  }
]

我试图得到这个结果:

[
  {
    "_id": null,
    "lead_count": 4
    "total_cost": 2.05
  }
]

我的聚合看起来像这样:

db.collection.aggregate([
  {
    "$match": {
      list_id: "5f76a53e8a93be772c14cc06"
    }
  },
  {
    "$group": {
      _id: null,
      total_cost: {
        $sum: "$cost"
      }
    }
  }
])

但它只给了我:

[
  {
    "_id": null,
    "total_cost": 2.05
  }
]

我需要如何调整它才能得到我想要的东西?以下是Playground链接: https://mongoplayground.net/p/zvLY67Oa_Ps