counters.go
func newAverageAccumulatingCounter(point Point) accumulatingCounter {
	return &averageAccumulatingCounter{
		newSingleValueAccumulatingCounter(point).(*singleValueAccumulatingCounter),
		point.(*CountPoint).Count.(float64),
	}
}
func newAverageAccumulatingCounter(point Point) accumulatingCounter {
	return &averageAccumulatingCounter{
		newSingleValueAccumulatingCounter(point).(*singleValueAccumulatingCounter),
		point.(*CountPoint).Count.(float64),
	}
}
// Average accumulating counter. Expect adding and removing CountPoints.