如何在库mgo中使用golang中的$facet？

package main

import (
    "fmt"
    "gopkg.in/mgo.v2"
    "gopkg.in/mgo.v2/bson"
)

type Result struct {
    ID    int    `bson:"_id"`
    Count int    `bson:"count"`
}

func main() {
    // 创建mgo.Session对象
    session, err := mgo.Dial("mongodb://localhost:27017")
    if err != nil {
        panic(err)
    }
    defer session.Close()

    // 创建mgo.Database对象
    db := session.Copy().DB("mydb")

    // 创建mgo.Pipe对象
    pipe := db.C("mycollection").Pipe([]bson.M{
        {
            "$facet": bson.M{
                "result1": []bson.M{
                    {"$match": bson.M{"field1": "value1"}},
                    {"$group": bson.M{"_id": "$field2", "count": bson.M{"$sum": 1}}},
                },
                "result2": []bson.M{
                    {"$match": bson.M{"field3": "value3"}},
                    {"$group": bson.M{"_id": "$field4", "count": bson.M{"$sum": 1}}},
                },
            },
        },
    })

    // 执行聚合操作
    var results []Result
    err = pipe.All(&results)
    if err != nil {
        panic(err)
    }

    // 处理结果
    for _, result := range results {
        fmt.Printf("ID: %d, Count: %d\n", result.ID, result.Count)
    }
}