Provide the Host value to handlers

[ymmt2005]

I wanted to use the Host field value of the http.Request in my application.
Since Go does not set Host header in http.Header, connect-go handlers cannot
access this information.

ref: https://cs.opensource.google/go/go/+/refs/tags/go1.20.4:src/net/http/request.go;l=157-158

Describe the solution you'd like
It would be great if connect.Request provided the Host field value in the http.Request.
For example,

func (MyHandler) DoSomething(ctx context.Context, req *connect.Request[pb.Request]) (*connect.Response[pb.Response], error) {
    fmt.Println(req.Host())
}

Describe alternatives you've considered
My current workaround is to wrap the HTTP handler returned by NewXXX generated function like:

	mux := http.NewServeMux()
	p, h := myserviceconnect.NewMyServiceHandler(handler)
	h2 := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		r.Header.Set("Host", r.Host)
		h.ServeHTTP(w, r)
	})
	mux.Handle(p, h2)

This way, I can access the Host value from handlers with req.Header().Get("Host").

[jhump]

It looks like the reverse flow is also problematic: the Go "net/http" client ignores a header key of "Host", instead requiring the caller to set the Host field of the *http.Request. But Connect clients don't have access to set this field. So in addition to currying the request.Host field into headers for server handlers, we should do the reverse (set the request.Host field from headers, if set by client or interceptor) on the client side.

[emcfarlane]

@ymmt2005 fixed in a similar way to the workaround. Host is available in the request header.

[ymmt2005]

@emcfarlane Thank you so much!