Add support to ignore request in metrics

[jan-stanek]

Changes

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

/kind api-change

Fixes #

Release Note

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

Docs

- [knative/docs]: https://github.com/knative/docs/pull/6177

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[knative-prow]

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

[knative-prow]

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[knative-prow]

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