code: public void serializeTo(CodedOutputStream output, int fieldNumber, K key, V value) throws IOException {
          output.writeTag(fieldNumber, WireFormat.WIRETYPE_LENGTH_DELIMITED);
          output.writeUInt32NoTag(computeSerializedSize(metadata, key, value));
          writeTo(output, metadata, key, value);
      }
docs: Serializes the provided key and value as though they were wrapped by a {@link MapEntryLite} to the output stream.
  This helper method avoids allocation of a {@link MapEntryLite} built with a key and value and is called from
  generated code directly.
  
  @param output the output
  @param fieldNumber the field number
  @param key the key
  @param value the value
  @throws IOException Signals that an I/O exception has occurred.