static public String popByKey(String key) {
        if (getMDCAdapter() == null) {
            throw new IllegalStateException(MDC_APAPTER_CANNOT_BE_NULL_MESSAGE);
        }
        return getMDCAdapter().popByKey(key);
    }
Pop the stack referenced by 'key' and return the value possibly null.

@param key identifies the deque(stack)
@return the value just popped. May be null/
@since 2.0.0
slf4j-api/src/main/java/org/slf4j/MDC.java