I was wondering the same thing.
I just went and looked at the source.
The javadoc says
"A service offering accumulator style counters to help in diagnosing performance issues."
It is registered at "java:/CounterService"
and has the following method
* accumulate a count into a named counter. will initialize the named
* counter if neccessary.
public void accumulate(String counterName, long add)...
The console will then display all the counters and values.
So, now we know.