Explicit Incremental Counter

Summary:
I didn't like how we forced anything registered with type
COUNTER to always report the difference between the last and current
value.  What if users want a COUNTER and know what they're doing.

To make this explicit, I've created IncrementalCounter that does
this, but otherwise behaves like a Counter to users.  I've also
invented the term IncrementalCounter to distinguish between
CumulativeCounter and Counter.

Test Plan: mvn test

Reviewers: kris

Reviewed By: kris