Enhance PSI example CPU stress pod; fix PSI Prometheus metrics grep regex

[roycaihw]

Description

Followup of #51541

Some enhancement and fix to the "understanding PSI" doc:

1. Add CPU resource requests and limits to the example CPU stress pod to produce more reliable CPU stress. This is a best practice that we're also adding to the k/k E2E test. Also see evidence of PSI results for pods without the limit v.s. with the limit. Users will observe more visible PSI data for the latter.
2. Fix PSI Prometheus metrics grep regex so that users can correctly query the metrics for the target containers. This was an oversight in the doc. See evidence of the actual metrics format.

Ref kubernetes/enhancements#4205

/cc @lmktfy @haircommander

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[reylejano]

@roycaihw , with dev-1.34 branch merged to main yesterday, can you change the target of this PR to main

[roycaihw]

@reylejano Thanks. Changed to main. PTAL :)

[roycaihw]

Based on slack communication (@aibarbetta), the PSI 1.34 blog is being scheduled to get published on Sep 4. Can we prioritize reviewing this PSI doc fix? I want to block the blog post on this doc fix. @lmktfy @haircommander @reylejano

[SergeyKanzhelev]

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: e9be6e6cea5814a24e20cdd3cf730b826e7b88e6

[dipesh-rawat]

/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dipesh-rawat, SergeyKanzhelev

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