compute/metadata: add retries

[codyoss]

Add retries consistent to what is being done in other languages.

[codyoss]

Can probably borrow the logic used in apiaries today. I think that should cover the different edge cases well enough: https://github.com/googleapis/google-api-go-client/blob/8258338b24dd72029fd1705ff1a5fa15d55ce17c/internal/gensupport/resumable.go#L233-L258

[hostirosti]

/sub

[codyoss]

@hostirosti This change is on HEAD. I will cut a release that includes this change on Monday, waiting for some other stuff to land first.