Create API for GCE Metadata

[JustinBeckwith]

There's a great metadata service for getting access to information on the VM. It would be great to have a structured API for this:
https://cloud.google.com/compute/docs/metadata?hl=en

[mziccard]

That would be nice indeed. I think we will investigate this as part of #85.
The only "issue" I see is that gcloud-java is meant to provide access to Google Cloud services from everywhere while the GCE metadata API is only available from inside a GCE VM, right?
But we can always add this support and document it's only available from a GCE VM (throws exception otherwise). How does this sound?

[dtretyakov]

It would be great to have such API to get rid of manual metadata retrieving and don't forget to support "wait-for-change" feature.

[garrettjonesgoogle]

Requests for new clients are being tracked internally. Closing out this issue.