private Set<MetricId> metricsToRetain() {
        return consumers.getConsumersByMetric().keySet().stream()
                .map(ConfiguredMetric::id)
                .collect(toCollection(LinkedHashSet::new));
    }
Handling of metrics received from external processes.

@author gjoranv
metrics-proxy/src/main/java/ai/vespa/metricsproxy/metric/ExternalMetrics.java