auth: add span to FetchToken helpers

[jedevc]

Before this, during a call to the docker resolver, we would generate span wrappers for each HTTPRequest correctly, however, as the docker resolver reaches out to the docker authorizer, it could create HTTP requests (for fetching tokens) that would not be wrapped in any span.

This can result in rather confusing traces, e.g. something like:

remotes.docker.resolver.HTTPRequest
	HTTP HEAD (fetch index, fails with 401)
HTTP GET (fetch token)
remotes.docker.resolver.HTTPRequest
	HTTP HEAD (fetch index)
remotes.docker.resolver.HTTPRequest
	HTTP GET (fetch manifest)

By adding a span into the FetchToken, this trace becomes a little easier to consume:

remotes.docker.resolver.HTTPRequest
	HTTP HEAD (fetch index, fails with 401)
remotes.docker.resolver.FetchToken
	HTTP GET (fetch token)
remotes.docker.resolver.HTTPRequest
	HTTP HEAD (fetch index)
remotes.docker.resolver.HTTPRequest
	HTTP GET (fetch manifest)

I think this is the right kind of approach for this. However, maybe it makes more sense to add this in the authHandler instead? Happy to discuss 🎉

[k8s-ci-robot]

Hi @jedevc. Thanks for your PR.

I'm waiting for a containerd member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[kzys]

/ok-to-test