[Doc] Update user guide for namespace label

[Electronic-Waste]

What you would like to be added?

Update user guide for the namespace label for metrics collector injection.

Why is this needed?

As we discussed in #2493 (comment), @andreyvelich suggested that the namespace label for metrics collector injection should not be patched in the Python SDK. We need to update the user guide to inform users of the effect of this label.

/cc @kubeflow/wg-automl-leads @helenxie-bit @tariq-hasan @Doris-xm @truc0

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[Electronic-Waste]

/remove-label lifecycle/needs-triage
/good-first-issue
/area docs

[google-oss-prow]

@Electronic-Waste:
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In response to this:

/remove-label lifecycle/needs-triage
/good-first-issue
/area docs

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[Doris-xm]

I'd like to help with it.
/assign

[Electronic-Waste]

/assign @Doris-xm