Timer rate mean and duration mean collide

[mcurcic]

In DatadogReporter.java in processTimer() the calls to processMeter() and pushSummarizable() export different types of mean to the same "mean" name.

My suggestion is to rename
pushGauge(name, meter.meanRate(), epoch, "mean");
to
pushGauge(name, meter.meanRate(), epoch, "meanRate");