flink sql读kafka元数据问题

[酷酷的浑蛋]

WITH("`event_time` TIMESTAMP(3) METADATA FROM 'timestamp'," +
"`partition` BIGINT METADATA VIRTUAL," +
"`offset` BIGINT METADATA VIRTUAL," +
"`headers` MAP<STRING, BYTES> NOT NULL METADATA VIRTUAL,”+
在这里获取kakfa元数据时，官网没有说怎么获取kafka消息的key？，headers的信息是空的，请问怎么在flink sql中获取kafka消息key？