Provide access to an instance's region from metadata

[prefanatic]

I've found myself carrying around a method to derive a region from metadata.google.internal. It's fairly analogous to projectIdFromMetadataServer from package:gcp

Usage of this method looks like:

final location = await locationFromMetadataServer();
print(location);
...
// Outputs: us-central1

I've scanned through a couple other issues, it looks like #123 is somewhat related, although is potentially larger in scope.

Would this be a welcome contribution, to be added to package:gcp?

[kevmoo]

Something for pkg:gcp sounds good to me!