private Map<MetricId, List<MetricId>> outputNamesById() {
        Map<MetricId, List<MetricId>> outputNamesById = new LinkedHashMap<>();
        for (ConfiguredMetric metric : consumers.getConsumersByMetric().keySet()) {
            outputNamesById.computeIfAbsent(metric.id(), unused -> new ArrayList<>())
                    .add(metric.outputname());
        }
        return outputNamesById;
    }
Returns a mapping from metric id to a list of the metric's output names.
Metrics that only have their id as output name are included in the output.
metrics-proxy/src/main/java/ai/vespa/metricsproxy/metric/ExternalMetrics.java