Setting static outbound IP address for App Engine(Flex)

I want to set a static outbound IP address for my App Engine instance, hence I went through this documentation. I followed all the steps but I am not able to see any traffic logged in the Cloud NAT for the IP address I specified in step 5 (Reserve a static IP address). I realized that this documentation is applicable for App Engine Standard environment and I deploy the instances of my application in App Engine Flex Environment. Does this documentation apply to App Engine Flex Environment as well? If not, how should I setup outbound static IP address for my App Engine flex instances?