public void LastValueAggregatorAggregatesCorrectly()
        {
            // create an aggregator
            LastValueAggregator <long> aggregator = new LastValueAggregator <long>();
            var sum = aggregator.ToMetricData() as SumData <long>;

            // we start with 0.
            Assert.Equal(0, sum.Sum);

            aggregator.Update(10);
            aggregator.Update(20);
            aggregator.Update(30);
            aggregator.Update(40);

            aggregator.Checkpoint();
            sum = aggregator.ToMetricData() as SumData <long>;
            Assert.Equal(40, sum.Sum);
        }