public MetricSet metricSet(MetricDimensions dim) {
        MetricSet metricSet = data.get(dim);
        if (metricSet == null) {
            data.put(dim, metricSet = new MetricSet());
        }
        return metricSet;
    }
Returns a metric set from this snapshot for a given set of dimensions
container-core/src/main/java/com/yahoo/container/jdisc/state/MetricSnapshot.java