@Override
    public Set<K> keySet() {
        var keys = new HashSet<>(secondary.keySet());
        keys.addAll(primary.keySet());
        return keys;
    }
A read-only map which forwards lookups to a primary map, and then a secondary for
keys not existing in the primary.

@author bratseth
container-search/src/main/java/com/yahoo/search/query/profile/ChainedMap.java