code: @Override
      protected Set<String> getKeySet() {
          HashSet<String> result = new HashSet<>();
  
          for (ConfigSource config : getConfigSources()) {
              Map<String, String> props = config.getProperties();
              if (props != null) {
                  Set<String> keys = props.keySet();
                  result.addAll(keys);
              }
          }
  
          return result;
      }
docs: {@inheritDoc}