@Override
    public ConfigInstance.Builder getConfig(ConfigInstance.Builder builder, String configId) {
        checkId(configId);
        Optional<ConfigProducer> configProducer = getConfigProducer(configId);
        if (configProducer.isEmpty()) return null;

        populateConfigBuilder(builder, configProducer.get());
        return builder;
    }
Resolves config for a given config id and populates the given builder with the config.

@param builder a configinstance builder
@param configId the config id for the config client
@return the builder if a producer was found, and it did apply config, null otherwise
config-model/src/main/java/com/yahoo/vespa/model/VespaModel.java