docs: public static String validateIndexAttribute(String attribute) {
          checkHasText(attribute, "Map index attribute must contain text");
          String keyPrefix = KEY_ATTRIBUTE_NAME.value();
          if (attribute.startsWith(keyPrefix) && attribute.length() > keyPrefix.length()) {
              if (attribute.charAt(keyPrefix.length()) != '#') {
                  LOG.warning(KEY_ATTRIBUTE_NAME.value() + " used without a following '#' char in index attribute '"
                          + attribute + "'. Don't you want to index a key?");
              }
          }
          return attribute;
      }