public Set<String> featureNames() {
        if (isEmpty()) return Set.of();
        Set<String> featureNames = new LinkedHashSet<>();
        if (encodedValues != null)
            encodedValues.fields().forEach(field -> featureNames.add(field.getKey()));
        if (values != null)
            featureNames.addAll(values.keySet());
        return featureNames;
    }
Returns the names of the features available in this
container-search/src/main/java/com/yahoo/search/result/FeatureData.java