code: public int getMessageSetSerializedSize() {
      int size = 0;
      for (int i = 0; i < fields.getNumArrayEntries(); i++) {
        size += getMessageSetSerializedSize(fields.getArrayEntryAt(i));
      }
      for (final Map.Entry<FieldDescriptorType, Object> entry :
               fields.getOverflowEntries()) {
        size += getMessageSetSerializedSize(entry);
      }
      return size;
    }
docs: Like {@link #getSerializedSize} but uses MessageSet wire format.