void add(MetricSnapshot snapshot) {
        for (Map.Entry<MetricDimensions, MetricSet> entry : snapshot) {
            MetricSet metricSet = data.get(entry.getKey());
            if (metricSet != null) {
                metricSet.add(entry.getValue());
            } else {
                data.put(entry.getKey(), entry.getValue());
            }
        }
    }
Returns all the metrics in this snapshot.
container-core/src/main/java/com/yahoo/container/jdisc/state/MetricSnapshot.java