public ConfigDef define(ConfigKey key) {
        if (configKeys.containsKey(key.name)) {
            throw new ConfigException("Configuration " + key.name + " is defined twice.");
        }
        if (key.group != null && !groups.contains(key.group)) {
            groups.add(key.group);
        }
        configKeys.put(key.name, key);
        return this;
    }
Returns unmodifiable set of properties names defined in this {@linkplain ConfigDef}

@return new unmodifiable {@link Set} instance containing the keys
clients/src/main/java/org/apache/kafka/common/config/ConfigDef.java