fix: custommetrics: always extract the headers but only write it when we have metrics

[chrischdi]

What this PR does / why we need it:

This PR removes the hasResources check to initialise all custom resource metric headers at startup.
By doing this, there will be no delay in adding custom resource metrics on startup (because of trying to list resources and timeout after 10s and then doing the same for the next bunch of CRs)

Instead the metrics writer now checks if there are are metrics in the first element of the store for the given header and outputs depending on that.

How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)

Does not affect the cardinality.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #2142

[mrueg]

Can you rename the commit message to be prefixed with: feat(custommetrics) or similar?

/assign @rexagod

[mrueg]

/triage accepted

[chihshenghuang]

i can confirm this PR fixed the first issue i mentioned at #2141 , thanks @chrischdi

[mrueg]

/lgtm

/hold
for others to review.

[rexagod]

/lgtm
This makes so much sense, thank you! 🎉

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chrischdi, mrueg, rexagod

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• OWNERS [mrueg,rexagod]

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[dgrisonnet]

/unhold