public Map<String, Map<String, String>> getExtraMapFields() {
    return extraMapFields;
  }
This can be used for extra fields that are not recognized by {@link DataNodeConfigSource} but still need to be
read from or written to the source of truth. This should be used sparingly and is mainly provided for legacy
compatibility.
@return a map from field name to map-style fields.
ambry-api/src/main/java/com/github/ambry/clustermap/DataNodeConfig.java