[release-0.22] allow unknown metadata fields

[knative-prow-robot]

This is an automated cherry-pick of #2249

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[dprotaso]

/retest

[dprotaso]

@googlebot I consent.

[markusthoemmes]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: knative-prow-robot, markusthoemmes

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