Support "arbitrary" mean throughput intervals in Meter

[CesarPantoja]

The class ExpWeightedMovingAvg has a parameter which allows you to specify the period of reporting, yet there are 3 hardcoded instances in Meter:

            self.m1rate = ExpWeightedMovingAvg(period=1, clock=self.clock)
            self.m5rate = ExpWeightedMovingAvg(period=5, clock=self.clock)
            self.m15rate = ExpWeightedMovingAvg(period=15, clock=self.clock)

Which results in only getting those metrics in the dashboard.

Would be great if you could also specify your own intervals. Something like Meter(clock=self._clock, [1, 5, 15, 30, 60])

Would this be useful? Seems fairly trivial to implement, would be happy to create the PR.

[omergertel]

Sounds legit, mind opening a PR?
Make the default so that behavior doesn't change? Not sure what to do with the getters though