Instrumented Per-Filter Processing Time Metrics, Implemented TimerByCategory Metrics

This work was completed to aid with B-57530 (Pull Request #726 from Jorge to fix RR inefficiencies in code).

The Yammer Metrics "Timer" class has been made accessible, and a corresponding TimerByCategory was created to make reporting grouped timers easier. The PowerFilterChain class was modified to report the processing time/delay of each filter (just as x-trace-request has in the past).