public List<Entry<String, String>> entries() {
        List<Entry<String, String>> list = new ArrayList<>(content.size());
        for (Entry<String, List<String>> entry : content.entrySet()) {
            String key = entry.getKey();
            for (String value : entry.getValue()) {
                list.add(new MyEntry(key, value));
            }
        }
        return List.copyOf(list);
    }
Returns an unmodifiable list of all key-value pairs of this. This provides a flattened view on the content of
this map.

@return the collection of entries
jdisc_core/src/main/java/com/yahoo/jdisc/HeaderFields.java