fix CPU time of pod stats with cs.CPU.Time

[qiutongs]

What type of PR is this?
/kind bug

What this PR does / why we need it:
The "stats update" time of pod CPU stats is incorrectly set to the container start time. This PR fixes this bug.

Which issue(s) this PR fixes:

Fixes #93988

Special notes for your reviewer:

• I took over the previous PR change cs.StartTime to cs.CPU.Time in addPodCPUMemoryStats #94034. There was some problem with the unit tests. After careful look, I find we need to mock a pod that cadviosr does not return stats info. Only by doing that can we reach the problematic code branch
• For the change of unit test, I verified the tests failed with old code.
• Here is how I run the unit tests

make test WHAT=./pkg/kubelet/stats/ GOFLAGS=-v

Does this PR introduce a user-facing change?:

NONE

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

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[SergeyKanzhelev]

/ok-to-test

[qiutongs]

/retest

[qiutongs]

I believe the failed test is not caused by this PR: //staging/src/k8s.io/apiserver/pkg/server/filters:go_default_test

[SergeyKanzhelev]

/test pull-kubernetes-bazel-test

[SergeyKanzhelev]

/priority important-soon

cc: @bobbypage

• After careful look, I find we need to mock a pod that cadviosr does not return stats info. Only by doing that can we reach the problematic code branch

So this is mostly theoretical issue then? I wonder how this statement alignes with this condition where it explicitly checks that the cs stats is NOT nil to enter this code branch.

	// Sum Pod cpu and memory stats from containers stats.
	if cs.CPU != nil {

[qiutongs]

/test pull-kubernetes-bazel-test

[SergeyKanzhelev]

I misunderstood code and @qiutongs explained it offline.

/lgtm

[qiutongs]

/assign @dashpole

[dashpole]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dashpole, qiutongs, ruiwen-zhao

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• pkg/kubelet/stats/OWNERS [dashpole]

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