code: public com.google.protobuf.ByteString
        getOldValueBytes() {
      java.lang.Object ref = oldValue_;
      if (ref instanceof java.lang.String) {
        com.google.protobuf.ByteString b = 
            com.google.protobuf.ByteString.copyFromUtf8(
                (java.lang.String) ref);
        oldValue_ = b;
        return b;
      } else {
        return (com.google.protobuf.ByteString) ref;
      }
    }
docs: <pre>
  Value of the changed object in the old Service configuration,
  in JSON format. This field will not be populated if ChangeType == ADDED.
  </pre>
  
  <code>string old_value = 2;</code>