code: @Override
      public Set<String> getKeys() {
          Map<String, String> map = inheritableThreadLocal.get();
          if (map != null) {
              return map.keySet();
          } else {
              return null;
          }
      }
docs: Returns the keys in the MDC as a {@link Set} of {@link String}s The
  returned value can be null.
  
  @return the keys in the MDC