trace: better error message for double init

Fixes golang/go#24137

Change-Id: Ifcf6449aed32071cdb643028347ec814bffd8d06
Reviewed-on: https://go-review.googlesource.com/127121
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

trace/trace.go

@@ -70,6 +70,7 @@ import (
 	"log"
 	"net"
 	"net/http"
+	"net/url"
 	"runtime"
 	"sort"
 	"strconv"
@@ -110,6 +111,13 @@ var AuthRequest = func(req *http.Request) (any, sensitive bool) {
 }
 
 func init() {
+	_, pat := http.DefaultServeMux.Handler(&http.Request{URL: &url.URL{Path: "/debug/requests"}})
+	if pat != "" {
+		panic("/debug/requests is already registered. You may have two independent copies of " +
+			"golang.org/x/net/trace in your binary, trying to maintain separate state. This may " +
+			"involve a vendored copy of golang.org/x/net/trace.")
+	}
+
 	// TODO(jbd): Serve Traces from /debug/traces in the future?
 	// There is no requirement for a request to be present to have traces.
 	http.HandleFunc("/debug/requests", Traces)