@Override
    public Config rewrite(String name, Config config) {
      Map<String, String> newConfigMap = new HashMap<>(config);
      if (config.containsKey(NEW_CONFIG_KEY)) {
        // for testing overwriting of new configs
        newConfigMap.put(NEW_CONFIG_KEY, config.get(NEW_CONFIG_KEY) + config.get(NEW_CONFIG_KEY));
      } else {
        newConfigMap.put(CONFIG_KEY, config.get(CONFIG_KEY) + config.get(CONFIG_KEY));
      }
      return new MapConfig(newConfigMap);
    }
If an entry at {@link #NEW_CONFIG_KEY} exists, overwrites it to be the value concatenated with itself. Otherwise,
updates the entry at {@link #CONFIG_KEY} to be the value concatenated to itself.
samza-core/src/test/java/org/apache/samza/util/TestConfigUtil.java