Metrics InterceptorBase uses SortedMap

[tomas-langer]

It seems the InterceptorBase uses SortedMap from API to get hold of a specific metric.
This should be refactored to use the registry method to get appropriate metric type,
such as MetricRegistry.counter(...) instead of MetricRegistry.getCounters(), as the first is implemented without a need for sorting all metrics every time a metric is needed.

This is a performance overhead.

[tjquinno]

The changes for issue #1595 addressed a couple of metrics interceptor performance issues, of which this was one. Marking this as closed.