Reporters: report Timer sums in the Timer duration units

[michaelpj]

Reporting sums for Timers is very useful, however they are currently always reported in nanoseconds, instead of in the Timer's duration units. This is because we inherit the handling of getSum from Meter, where it has a slightly different meaning.

There are a couple of unsatisfying bits:

• JmxTimer loses some precision in reporting the sum, because we want to be able to report fractional values, but JmxMeter assumes that it can be represented as a long. We could probably just change JmxMeter to report a double.
• GraphiteReporter.reportTimer delegates to a reportMetered, which will do the wrong thing with the sum.

[arteam]

Thanks! This is a very nice PR and I agree that for Timers we should report the sum in a reporter's duration unit. Regarding your questions:

• I think it makes sense to change getSum of JmxMeterMBean to double, because it will allow to report sums more precise in the reporter's duration unit. It's a new field, so we don't have any requirements to keep compatibility.

• In GraphiteReporter we could add a new parameter to the reportMetered method and check whether we shoud convert it:

 private void reportMetered(MetricName name, Metered meter, long timestamp, boolean isSumADuration) throws IOException {
        sendIfEnabled(COUNT, name, meter.getCount(), timestamp);
        sendIfEnabled(SUM, name, isSumADuration ? convertDuration(meter.getSum()) : meter.getSum(), timestamp);

I would gladly review both changes as separate PRs.

[michaelpj]

Thanks for the quick review! I've put up the suggested follow-up PRs.