code: function addMetadataValue(name, value) {
    if (typeof name !== 'string' || typeof value !== 'string' ) {
      throw new Error('metadataValues.add() takes a string name and string value');
    }
    var metaVal = {};
    metaVal[name] = value;
    return insert('/object-node("metadataValues")', 'last-child', metaVal);
  }
docs: Specifies operations to patch the metadata values of a document.
  @namespace patchBuilderMetadataValues
  @since 2.0.1
   
  Specifies a new metadata value to add to a document.
  @method patchBuilderMetadataValues#add
  @since 2.0.1
  @param {string} name - the name of the new metadata value
  @param value - the value of the new metadata value
  @returns {patchBuilder.PatchOperation} a patch operation