@Override
    public Deque<String> getCopyOfDequeByKey(String key) {
        return threadLocalMapOfDeques.getCopyOfDequeByKey(key);
    }
Returns the keys in the MDC as a {@link Set}. The returned value can be
null.
zlt-commons/zlt-log-spring-boot-starter/src/main/java/org/slf4j/TtlMDCAdapter.java