private void addSetting(Index index, RankSetting.Type type, Set<RankSetting> settings) {
        RankSetting setting = toRankSetting(index, 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