public ConfigProducer getConfigProducer(String configId) {
        return id2producer.get(configId);
    }
Returns the ConfigProducer with the given id, or null if no such
configId exists.

@param configId The configId, e.g. "search.0/tld.0"
@return ConfigProducer with the given configId
config-model/src/main/java/com/yahoo/config/model/ApplicationConfigProducerRoot.java