code: public synchronized JaxRsClientFactory addFeatureToGroup(JaxRsFeatureGroup group, Feature... features) {
          Preconditions.checkState(!started, "Already started building clients");
          featureMap.putAll(group, Arrays.asList(features));
          LOG.trace("Group {} registers features {}", group, features);
          return this;
      }
docs: Register a list of features to all clients marked with the given group.