Add FetchMigActualSize method to GCE

[tetianakh]

What type of PR is this?

/kind bug

What this PR does / why we need it:

This change adds a way to get the actual size of a node group without listing its instances. ListManagedInstances is an expensive API call and if we only need the number of nodes we can use nodeGroups.get.

Does this PR introduce a user-facing change?

None

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: tetianakh
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• cluster-autoscaler/cloudprovider/gce/OWNERS

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[k8s-ci-robot]

Hi @tetianakh. Thanks for your PR.

I'm waiting for a kubernetes 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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[jackfrancis]

/ok-to-test