Support METADATA operator in header mutation

[guydc]

Title: Support METADATA operator in header mutation

Description: The METADATA formatter operator, provided by the metadata formatter extension is planned to replace existing built-in access log operators such as DYNAMIC_METADATA, CLUSTER_METADATA, etc. The METADATA operator should also be supported in the header value of the route *_headers_to_add feature.

Currently, envoy rejects the METADATA operator in header values with the following error: Not supported field in StreamInfo: METADATA, while operators like DYNAMIC_METADATA, CLUSTER_METADATA are supported.

[mathetake]

https://github.com/envoyproxy/envoy/compare/main...mathetake:envoy:metadatahttpmutation?expand=1 just quickly created a WIP branch for this

[github-actions[bot]]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or "no stalebot" or other activity occurs. Thank you for your contributions.

[github-actions[bot]]

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