feat: Add ability to echo request headers in trailing metadata

[parthea]

This includes fixes from #1500. Please review #1500 first Done

Fixes #1493

Using this showcase server, I was able to see the http request headers in the trailing metadata

Response

responses.trailing_metadata()[0]
_Metadatum(key='x-goog-api-version', value='v1_20240408')

[parthea]

This works for gRPC. I haven't figured out how to get the information for REST

[parthea]

@noahdietz Please could you take another look? Should we also support this for REST? Can that be added in a separate PR?

[noahdietz]

Should we also support this for REST?

Yes, definitely.

Can that be added in a separate PR?

Yes, definitely.

We may want the generated REST handlers, like this:

gapic-showcase/server/genrest/echo.go

Lines 87 to 99 in a34fc37

	response, err := backend.EchoServer.Echo(ctx, request)
	if err != nil {
	backend.ReportGRPCError(w, err)
	return
	}
	json, err := marshaler.Marshal(response)
	if err != nil {
	backend.Error(w, http.StatusInternalServerError, "error json-encoding response: %s", err.Error())
	return
	}
	w.Write(json)

...to pull the same metadata from the context (as the gRPC handlers do) and write them as HTTP headers. I'd probably add a helper here for copying context metadata into HTTP response headers, then invoke that in the spot above (generator code here).

@vchudnov-g may be interested in this.

[noahdietz]

Please revert all of the changes in the server/genproto directory, the headers will just be replaced when things are regenerated by CI.

[parthea]

Done