In GCM was feature to create new API key, so on my current project different keys were used for staging and prod environment. But now according to info from here new keys created for GCM are not valid.
How can I create few FCM keys for one project?
For GCM or FCM, you'll be needing a Server Key. Steps are below on how to Create a New or Import a project to have a valid Server Key.
If you are just starting to use GCM, instead of creating a project in the Google Developers Console, do it in the Firebase Console. After creating the project, simply use the auto-generated Server Key. Here are the steps where to find the Server Key:
For old GCM projects, you can simply Import the project to the Firebase Console:
Portion taken from my answer here.
To add upon needing separate keys for both development and prod, once you create a new firebase project for your other environment, you'll have a new google-services.json file to incorporate with the new keys. At this point you can have your production google-services.json file in the app/src/release directory to keep them separate.
More info here:
https://developers.google.com/android/guides/google-services-plugin