public <T extends ConfigInstance> ConfigHandle<T> subscribe(Class<T> configClass, String configId) {
        return subscribe(configClass, configId, source, new TimingValues());
    }
Subscribes on the given type of {@link ConfigInstance} with the given config id.

The method blocks until the first config is ready to be fetched with {@link #nextConfig(boolean)}.

@param configClass The class, typically generated from a def-file using config-class-plugin
@param configId Identifies the service in vespa-services.xml, or null if you are using a local {@link ConfigSource} which does not use config id.
Also supported: raw:, file:, dir: or jar: config id which addresses config locally in the same way.

@return a ConfigHandle
config/src/main/java/com/yahoo/config/subscription/ConfigSubscriber.java