static MetricsConsumer addMetrics(MetricsConsumer original, Map<String, Metric> metrics) {
        if (metrics == null) return original;

        Map<String, Metric> combinedMetrics = new LinkedHashMap<>(original.metrics());
        metrics.forEach((name, newMetric) ->
                                combinedMetrics.put(name, combineMetrics(original.metrics().get(name), newMetric)));

        return new MetricsConsumer(original.id(),
                                   new MetricSet(original.metricSet().getId(), combinedMetrics.values()));
    }
@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