Directpath support on non prod environments #14246

Issue Details

Directpath does not work unless a google universe domain is in the URL. However, there are cases where directpath can be used in non-prod environments.

https://github.com/googleapis/google-cloud-java/blob/main/sdk-platform-java/gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java#L877.

Can we override this check with a option or environment variable so DP is available in non-prod?

See https://b.corp.google.com/issues/479544840#comment6 for details