Make MetricRegistry more extensible

This patch makes MetricRegistry.getOrAdd protected, improving extensibility of the Registry

Use case: I have a large number of short lived metrics. I created a registry that removes all unused metrics. This requires me to override getOrAdd (see https://github.com/wouterdb/metrics/blob/3.0/metrics-core/src/main/java/com/yammer/metrics/DecayingRegistry.java)