Add Stats collector service for Scaler.

 Currently reports list of nodes with capacity - memory and cpu.

Periodically collects and reports  mean, max, and 90p usage stats for memory and cpu.
I think we need the float computation here to get the right index. Is there a typo in your suggestion:
100/90 is just 1.  If you multiply by count first, you'll end up with an invalid index (for count==9, it will return 10).

Also, I need to use the fraction part in the next line. This helps if we land up between two buckets with huge count difference. 90p is 12.5 - 12th bucket is 100, 13th is 500. 

For floats, we mostly want to avoid them in user-facing API.