Add support to ignore request in metrics

[jan-stanek]

Fixes #14581, #15646

Proposed Changes

• Added possibility to mark request to be ignored in metrics using header K-Ignore-Metrics

Release Note

Added `K-Ignore-Metrics` header to ignore request in metrics.

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[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: jan-stanek
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[knative-prow]

Welcome @jan-stanek! It looks like this is your first PR to knative/serving 🎉

[knative-prow]

Hi @jan-stanek. Thanks for your PR.

I'm waiting for a knative member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[skonto]

You can test this in this PR but has to be moved to the related repo: https://github.com/knative/networking

[jan-stanek]

PR created: knative/networking#1025