@Override
    public <T extends ConfigInstance> ConfigHandle<T> subscribe(SingleSubscriber<T> singleSubscriber, Class<T> configClass, String configId) {
        throw new UnsupportedOperationException();
    }
Subscribes to config without using the class. For internal use in config proxy.

@param key the {@link ConfigKey to subscribe to}
@param defContent the config definition content for the config to subscribe to
@param timingValues {@link TimingValues}
@return generic handle
config/src/main/java/com/yahoo/config/subscription/impl/GenericConfigSubscriber.java