public void subscribe() {
        if (configURI != null) {
            subscriber = new ConfigSubscriber(configURI.getSource());
            subscriber.subscribe(this, MessagebusConfig.class, configURI.getConfigId());
        }
    }
Start listening for config updates. This method will not return until the handler has been configured at least
once unless an exception is thrown.
messagebus/src/main/java/com/yahoo/messagebus/ConfigAgent.java