code: func TransactionAddAttribute(id int64, name, value string) (int, error) {
  	cname := C.CString(name)
  	defer C.free(unsafe.Pointer(cname))
  
  	cvalue := C.CString(value)
  	defer C.free(unsafe.Pointer(cvalue))
  
  	return errNo(C.newrelic_transaction_add_attribute(C.long(id), cname, cvalue))
  }
docs: /*
   * Set a transaction attribute. Up to the first 50 attributes added are sent
   * with each transaction.
   *
   * @param transaction_id  id of transaction
   * @param name  attribute name
   * @param value  attribute value
   * @return  0 on success, else negative warning code or error code
   */