#2 Histogram stream - support non-integer count value

[Hai Chuan Wang]

The current histogram stream can only accept category value with integer count value.

However, in many cases, it need a histogram to calculate non-integer count value. E.g. calculating a CPU utilization time distribution.

Although we can use integer to simulate it, it's be nice if the category value can accept double type count, which makes it more intuitionisive.