The new SampleQuantiles class is useful in the context of benchmarks, but currently there is no way to print it out outside the context of a metrics sink. It would be nice to have a convenient way to stringify it for logging, etc.
- made it Comparable and changed the HashMap to TreeMap so that the printout is in ascending percentile order. Given that this map is always very small, and snapshot() is only called once a minute or so, the runtime/memory differences between treemap and hashmap should be negligible.
- changed the behavior to return null instead of throw, because all the catching, etc, got pretty ugly. In implementing toString, I figured I'd clean up the other behavior along the way.