code: public Map<String, String> getDetails() {
          if (details == null) {
              return null;
          }
          return Collections.unmodifiableMap(details);
      }
docs: Optional additional details about this message. This could be null if there are no additional details associated
  with this message.
  
  @return the details of this message or null. This is an unmodifiable, read-only map of details.