I was thinking that this way we can define 'what's of interest' -- i.e. if we have individual samplers for each actor, we have individual counters, but otherwise, it's a high-level config, so we're probably not poking one actor in particular. Also, it seemed to let me avoid using the ConcurrentHashMap, since it allows definition of the keys in the constructor... 
... think I'm probably getting a bit too defensive, though. So: change would be to use 'putIfAbsent(AtomicLong(0))' on the ConcurrentHashMap and use the actor as the key instead of the filter.