public CounterQuery addCounterFilter(String counter) {
   CounterFilterPredicate p = CounterFilterPredicate.fromString(counter);
   if (p == null) {
     return this;
   }
   if (counterFilters == null) {
     counterFilters = new ArrayList<CounterFilterPredicate>();
   }
   counterFilters.add(p);
   return this;
 }