Provide access to an instance's region from metadata

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:
```dart
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`?