add metrics for function duration for listing gce instances and metrics for inconsistent mig count when gce bulk instance listing is enabled

[damikag]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR emits function duration metrics for listing all mig instances and listing instances using igm.ListManagedInstances(), when bulk listing gce instances is enabled.

And also emits inconsistent mig count for the number of migs which have a difference in instance count based on actionsSummary and result from bulk listing instances.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

Hi @damikag. Thanks for your PR.

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[bskiba]

/lgtm
/approve

[x13n]

Looks good, just minor comments.

[x13n]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bskiba, damikag, x13n

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