Right now they're in [finagle-core][0], although we're trying to move them into a util-stats library, it's slow going since we want to keep the c.t.f.s.Counter abstractions compiling.

Maybe the right thing to do is to just make people opt into the fast counter explicitly.  How about you have to construct a `LongAdderCounter` via Stats.getFastCounter if you want fast stats?  It's a bummer because it means that old users don't just get magically faster, but it preserves correctness.

[0]: https://github.com/twitter/finagle/blob/master/finagle-core/src/main/scala/com/twitter/finagle/stats/StatsReceiver.scala#L10