code: static <K, V> void writeTo(CodedOutputStream output, Metadata<K, V> metadata, K key, V value) throws IOException {
          CodedConstant.writeElement(output, metadata.keyType, KEY_FIELD_NUMBER, key);
          CodedConstant.writeElement(output, metadata.valueType, VALUE_FIELD_NUMBER, value);
      }
docs: Write to.
  
  @param <K> the key type
  @param <V> the value type
  @param output the output
  @param metadata the metadata
  @param key the key
  @param value the value
  @throws IOException Signals that an I/O exception has occurred.