public Set<String> getKeys() {
        Map<String, String> readOnlyMap = getPropertyMap();

        if (readOnlyMap != null) {
            return readOnlyMap.keySet();
        } else {
            return null;
        }
    }
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