public void addDescendant(AnyConfigProducer descendant) {
        id2producer.put(descendant.getConfigId(), descendant);
    }
Adds the descendant (at any depth level), so it can be looked up
on configId in the Map.

@param descendant The configProducer descendant to add
config-model/src/main/java/com/yahoo/config/model/ApplicationConfigProducerRoot.java