MongoDB不同结构统计嵌套文档数?

 {
            "_id" : ObjectId("5cd8609db20663a19e2c362c"),
            "2017" : {
                "JAN" : [
                    {
                        "SYMBOL" : "20MICRONS",
                        "SERIES" : "EQ",
                        "OPEN" : "33.4",
                        "HIGH" : "34.3",
                        "LOW" : "33",
                    },
                    {
                        "SYMBOL" : "3IINFOTECH",
                        "SERIES" : "EQ",
                        "OPEN" : "5.8",
                        "HIGH" : "5.8",
                        "LOW" : "5.55",
                    },
                    {
                        "SYMBOL" : "3MINDIA",
                        "SERIES" : "EQ",
                        "OPEN" : "11199.9",
                        "HIGH" : "11233",
                        "LOW" : "10861",
                    }
                ]
            }
        }