code: @Override
  	public Collection<Object> getMeasureKeys() {
  		HashSet<Object> keys = new HashSet<>();
  		keys.addAll(pullers.keySet());
  		keys.addAll(pushers.keySet());
  		return keys;
  	}
docs: Provides the keys of all the {@link Measure}s which are supported by this
  {@link SimpleMeasureManager}. If a key is provided, then at least one
  version is available through {@link #getPullMeasure(Object)} or
  {@link #getPushMeasure(Object)}.