private static Metric combineMetrics(Metric original, Metric newMetric) {
        return original != null ? newMetric.addDimensionsFrom(original) : newMetric;
    }
@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