`headersDistinct` for http2 requests

[nomagick]

What is the problem this feature will solve?

headersDistinct is missing form http2 requests compatibility API.

Which makes it not super compatible.

What is the feature you are proposing to solve the problem?

headersDistinct for http2 requests

What alternatives have you considered?

No response

[Shivay-98]

I'd like to work on this.

My understanding is that headersDistinct is part of the http.IncomingMessage API, but it is currently missing from the HTTP/2 compatibility request implementation. This makes it harder to write protocol-agnostic middleware against the compatibility layer.

My proposal would be to add headersDistinct to Http2ServerRequest and have it mirror the semantics of IncomingMessage.headersDistinct, preserving multiple values instead of joining them.

Before opening a PR, I wanted to clarify one detail: should HTTP/2 pseudo-headers (:method, :path, :scheme, :authority) be included in headersDistinct, or should the behavior follow whatever is currently exposed through req.headers in the compatibility API?

Assuming that's the intended direction, I can put together an implementation and accompanying parity tests for HTTP/1 and HTTP/2 compatibility requests.