file https://github.com/googleapis/google-auth-library-python/blob/master/google/auth/environment_vars.py
contains GCE_METADATA_ROOT added in:
b33b802

Client libs for other languages call it GCE_METADATA_HOST:
go: https://github.com/googleapis/google-cloud-go/blob/master/compute/metadata/metadata.go
java: https://github.com/googleapis/google-auth-library-java/blob/master/oauth2_http/java/com/google/auth/oauth2/ComputeEngineCredentials.java
...

Maybe consider supporting both names for a transition?