How should I set metric parameter on Python facebook_business SDK

It looks Python facebook_business SDK does not have metric values. https://github.com/facebook/facebook-python-business-sdk/blob/master/facebook_business/adobjects/insightsresult.py

So I set a metric parameter by str though,

params = {'metric' : ['page_fan_adds_unique','page_fan_adds'], 'period': 'day'}

however it makes UserWarning.

.../lib/python3.7/site-packages/facebook_business/utils/api_utils.py:30: UserWarning: value of metric might not be compatible.  Expect list<Object>;  got <class 'list'>
  warnings.warn(message)

How can I resolve it?