Revert "- Added a notifyUsage() template method on FilterProcessor, so that the filter metrics implementation can be overridden."

This reverts commit 7379aa8425db941b746ce7d6df8e1ead811af254.
- Reverted the part of the previous commit, as changing from the Singleton pattern for FilterProcessor impacted too many filters that depended on getting access to it.
- Changed to allowing the filter usage handler to be overriden by injecting an alternate impl of the new FilterUsageNotifier interface.