meter by status level (1xx, 2xx, etc.)

This optionally adds new meters, for each level of statuses (1xx, 2xx, etc.), which allows the user to quickly see how many successful, failure, etc. requests have happened. 

Also makes the statuses to filter ("knownStatuses") user-configurable.

 Usage is controlled by config flags, and in the flag's absence, the old behavior is retained.