update stats APIs to use TimePoint vs Duration correctly

update stats APIs to use TimePoint vs Duration correctly

Summary:
Update the stats APIs to correcly distinguish between TimePoint and Duration
types.

This does leave addValue() and update() APIs in place that accept Duration
values, for backwards compatibility.  These should eventually be removed once
all code has been converted to call the new APIs.

Reviewed By: yfeldblum

Differential Revision: D3808805

fbshipit-source-id: 36d6574ba4a09db7eb9f1a35e47addd3e07f8461