private void addSetting(ImmutableSDField field, RankSetting.Type type, Set<RankSetting> settings) {
        if (type.isIndexLevel()) {
            addIndexSettings(field, type, settings);
        }
        else {
            RankSetting setting = toRankSetting(field, type);
            if (setting == null) return;
            settings.add(setting);
        }
    }
Returns the names of the fields which have a rank boost setting
explicitly in this profile or in fields
config-model/src/main/java/com/yahoo/schema/DefaultRankProfile.java