Add duration metric type.
incr is more on spot for pings.

Firstly, for timing statsd creates 7 derived metrics that need a time
duration value to make sense, not time.time(). Second the derived metrics
is usually sent more seldom than once per flush interval, and the derived
metrics aren't of much use with no more than one value per flush interval.
The event time is recorded by statsd, it's redundant and confusing to send
it as the metric value as well.
Bump version.