fix(metrics): make prometheus labels more type safe

[mwmix]

What does it do ?

Ensure labels are more type safe and applied in the correct order.

Motivation

While working on PR #5665 it was determined that we could use more type safety for prometheus labels.

More

• [ x ] Yes, this PR title follows Conventional Commits
• [ x ] Yes, I added unit tests
• [ x ] Yes, I updated end user documentation accordingly

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[mloiseleur]

/ok-to-test

[ivankatliarchuk]

Looks nice. We may going to have a higher chances of cyclic dependencies, as pkg/http depends on pkg/metrics, but something we could re-org when required.

TODO: to review when merged prometheus/client_golang#1598

/lgtm

[mloiseleur]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mloiseleur

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[redbaron]

@mwmix, @ivankatliarchuk , I think this change triggers race condition:

1. CustomRoundTripper::RoundTrip updates global var RequestDurationLabels
2. RoundTrip is called from http client and there can be multiple of them in parallel

Exact error I get is:

on b31afb4