private static MetricsConsumer combineConsumers(MetricsConsumer original, MetricsConsumer overriding) {
        if (overriding == null) return original;
        return addMetrics(original, overriding.metrics());
    }
@param userConsumers the consumers set up by the user in services.xml
@return a list of consumer builders (a mapping from consumer to its metrics)
config-model/src/main/java/com/yahoo/vespa/model/admin/metricsproxy/ConsumersConfigGenerator.java