Constant VALID_LOG_LEVELS 

pub const VALID_LOG_LEVELS: [&str; 6];

The level names a BROKER_LOGGER value may carry, in the order Kafka renders them into its rejection message.

RuntimeLoggerManager builds that list as String.join(", ", sorted(LogLevelConfig.VALID_LOG_LEVELS)), so a caller joins these with ", " and not with a bare comma.