There are no custom metrics nor custom dimensions in App + Web per se, however, there are equivalent called parameters and user properties.
Where it used to be a hit-scoped custom dimension, you will utilise parameters in the App+Web property and for user-scope dimensions, you will utilize user properties. It must be noted that user properties will require a code to be injected into the web/app.
For App + Web, you can send up to 25 custom parameters per hit sent and similar to current Google Analytics set up, custom parameters are not retroactive which means that it will only collect data once the parameter is enabled.
Currently there doesn’t seem to be the same sort of access to configure parameters like the dimension and metric setup within the ‘Property Settings’ in the UA version. This is possibly due to it not being available yet or Parameters being limited to Hit and User scopes and not requiring setup. See below for User Properties where configuration is possible.
What parameters are configured to send to the property?
To understand what parameters are being set you will need to navigate to your GTM setup and look in there. Under each tag – like in the screenshot for the 404 error event below – you can see which parameters are being set.
Though there are user properties that can be configured. These are the same as User Scoped Custom Dimensions that you find in Universal Analytics.
User Properties can be found under the Configure Section in the left hand side panel
There are just two configuration options per User Property – Name and Description.
Note User Property Names can’t have spaces and descriptions are limited to 150 characters.