MongoDB.Driver.Builders cómo agrupar y obtener un promedio

con el nuevo controlador MongoDB .NET (2.0 - http://docs.mongodb.org/ecosystem/drivers/csharp/), el soporte de Linq es totalmente compatible, aquí está la sintaxis de la pregunta usando el nuevo controlador. Código .NET mucho más legible que antes de usar la sintaxis de BsonDocument.

    public async Task<List<DailyStat>> GetLast31DaysReport(string id)
    {
        var mc = new MongoClient(_url);
        var db = mc.GetDatabase(DbName);
        var collection = db.GetCollection<Reading>(CollectionName);

        DateTime from = DateTime.Now.AddDays(-31);
        DateTime to = DateTime.Now;

        var output = await collection.Aggregate()
            .Match(r => r.SensorId == id)
            .Match(r => r.Date <= to)
            .Match(r => r.Date >= to.AddDays(-31))
            .Group(r => new { groupedYear = r.Date.Year, groupedMonth = r.Date.Month, groupedDay = r.Date.Day }, g =>
                new {
                    Key = g.Key,
                    avgValue = g.Average(x => x.Value),
                    minValue = g.Min(x => x.Value),
                    maxValue = g.Max(x => x.Value)
                })
            .Project(r => new DailyStat()
                {
                    Day = r.Key.groupedDay,
                    Month = r.Key.groupedMonth,
                    Year = r.Key.groupedYear,
                    Value = r.avgValue,
                    MinValue = r.minValue,
                    MaxValue = r.maxValue
                })
            .ToListAsync().ConfigureAwait(false);

        var returnList = new List<DailyStat>();
        while (returnList.Count < 31)
        {
            var value = output.FirstOrDefault(rec => rec.Day == from.Day && rec.Month == from.Month && rec.Year == from.Year);
            returnList.Add(value ?? new DailyStat() { Month = from.Month, Year = from.Year, Day = from.Day, Value = 0, MaxValue = 0, MinValue = 0 });
            from = from.AddDays(1);
        }
        return returnList;
    }