perf: make dispatcher loop stats work with statsd

I've been debugging a perf problem at Lyft and the dispatcher loop stats are indeed incredibly useful. We should make these stats usable with statsd. The best way I can see to do this is to potentially have the ability to tag a histogram as "high rate" so that it gets ignored by statsd for individual flushes, and then statsd could potentially flush high rate histograms as gauges with various percentiles dumped?