feat: support auto-instrumentation when net/http.DefaultServeMux is used implicitly

[keisku]

Motivation

orchestrion doesn't do auto-instrumentation for this code because handler in http.ListenAndServe() is set to nil. Setting nil in http.ListenAndServe() is generally a common practice.

package main

import (
	"context"
	"log/slog"
	"net/http"
	"os"
	"os/signal"
)

func main() {
	ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt)
	defer cancel()
	http.HandleFunc("/hello", func(w http.ResponseWriter, r *http.Request) {
		slog.InfoContext(r.Context(), "Request received at /hello")
		resp, err := http.Get("https://example.com")
		if err != nil {
			http.Error(w, err.Error(), http.StatusInternalServerError)
			return
		}
		defer resp.Body.Close()
		slog.InfoContext(r.Context(), "Response received from example.com", slog.String("status", resp.Status))
		w.WriteHeader(http.StatusOK)
	})
	slog.Info("Starting server on port 8080")
	go http.ListenAndServe(":8080", nil)
	<-ctx.Done()
	slog.Info("Server stopped")
}

Workaround

Set http.DefaultServeMux to http.Server.Handler explicitly.

+	s := http.Server{
+		Addr:    ":8080",
+		Handler: http.DefaultServeMux,
+	}
+	go s.ListenAndServe()
-	go http.ListenAndServe(":8080", nil)

Generated code

srv.Handler is nil, when we set nil in http.ListenAndServe().

func (srv *Server) Serve(l net.Listener) error {
	//line <generated>
	{
		if srv.Handler == nil {
			srv.Handler = __dd_orchestrion_instrument_WrapHandler(DefaultServeMux)
		} else {
			srv.Handler = __dd_orchestrion_instrument_WrapHandler(srv.Handler)
		}
	}
	//line /snap/go/10743/src/net/http/server.go:3301
	if fn := testHookServerServe; fn != nil {
		fn(srv, l) // call hook with unwrapped listener
	}
	// ...skip....

How I checked generated code? Use fmt.Println(buf.String()) in /internal/injector/write.go

Replace github.com/DataDog/orchestrion with the project in my local.

module orchestrion-poc

go 1.23.3

require github.com/DataDog/orchestrion v1.0.1

replace github.com/DataDog/orchestrion => /home/ubuntu/workspace/orchestrion
....skip...