Support for Request MetadataKind in InternalUpstreamTransport

Title: Support for Request MetadataKind in InternalUpstreamTransport

Description:

I have a usecase where I am extracting an http header in a Lua filter, setting it in dynamicMetadata and passing it to an internal listener which has a TCPProxy filter configured with tunneling_config. The value that is set in dynamicMetadata needs to be referenced in the tunneling_config.

It seems there is support for passthrough_metadata for passing information from one listener to the other listener in this case. However, even though the docs seem to suggest that request metadataKind is supported ( which is needed for passing DynamicMetadata ), it seems the internal implementation is missing here and here

Is my understanding correct ? Happy to work on a PR if I can get any pointers!

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