Locked down implementation of MeasurementReporter means you cannot add your own overrides for certain metric types

[lfoxton]

Please see #60 as a potential fix to this problem

Background for the issue is that influx/grafana reporting for timers reports timer datapoints regardless of whether there has been a metric recorded in the previous time frame block. Making TimerMeasurementReporter extensible allows me to add my own implementation that fixes this issue