Do we need to export all the methods?   I wonder if we can avoid exporting methods that nobody is using in order to limit the liability of maintaining backwards compatibility.  Also, this might be a good time to break the API and pick one of Duration or Timing since they basically do the same thing (I'd vote for Duration since it takes a time.Duration).

Filterdrecv (via sglog.MetricLogger) only uses SetPrefix, Increment, Timing (although we could change to Duration), and Gauge.