Improve queue & process & stacktrace (#24636)

Although some features are mixed together in this PR, this PR is not
that large, and these features are all related.

Actually there are more than 70 lines are for a toy "test queue", so
this PR is quite simple.

Major features:

1. Allow site admin to clear a queue (remove all items in a queue)
* Because there is no transaction, the "unique queue" could be corrupted
in rare cases, that's unfixable.
* eg: the item is in the "set" but not in the "list", so the item would
never be able to be pushed into the queue.
* Now site admin could simply clear the queue, then everything becomes
correct, the lost items could be re-pushed into queue by future
operations.
3. Split the "admin/monitor" to separate pages
4. Allow to download diagnosis report
* In history, there were many users reporting that Gitea queue gets
stuck, or Gitea's CPU is 100%
    * With diagnosis report, maintainers could know what happens clearly

The diagnosis report sample:
[gitea-diagnosis-20230510-192913.zip](https://github.com/go-gitea/gitea/files/11441346/gitea-diagnosis-20230510-192913.zip)
, use "go tool pprof profile.dat" to view the report.


Screenshots:


![image](https://github.com/go-gitea/gitea/assets/2114189/320659b4-2eda-4def-8dc0-5ea08d578063)


![image](https://github.com/go-gitea/gitea/assets/2114189/c5c46fae-9dc0-44ca-8cd3-57beedc5035e)


![image](https://github.com/go-gitea/gitea/assets/2114189/6168a811-42a1-4e64-a263-0617a6c8c4fe)

---------

Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: Giteabot <teabot@gitea.io>

routers/web/admin/diagnosis.go

@@ -0,0 +1,61 @@
+// Copyright 2023 The Gitea Authors.
+// SPDX-License-Identifier: MIT
+
+package admin
+
+import (
+	"archive/zip"
+	"fmt"
+	"runtime/pprof"
+	"time"
+
+	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/httplib"
+)
+
+func MonitorDiagnosis(ctx *context.Context) {
+	seconds := ctx.FormInt64("seconds")
+	if seconds <= 5 {
+		seconds = 5
+	}
+	if seconds > 300 {
+		seconds = 300
+	}
+
+	httplib.ServeSetHeaders(ctx.Resp, &httplib.ServeHeaderOptions{
+		ContentType: "application/zip",
+		Disposition: "attachment",
+		Filename:    fmt.Sprintf("gitea-diagnosis-%s.zip", time.Now().Format("20060102-150405")),
+	})
+
+	zipWriter := zip.NewWriter(ctx.Resp)
+	defer zipWriter.Close()
+
+	f, err := zipWriter.CreateHeader(&zip.FileHeader{Name: "goroutine-before.txt", Method: zip.Deflate, Modified: time.Now()})
+	if err != nil {
+		ctx.ServerError("Failed to create zip file", err)
+		return
+	}
+	_ = pprof.Lookup("goroutine").WriteTo(f, 1)
+
+	f, err = zipWriter.CreateHeader(&zip.FileHeader{Name: "cpu-profile.dat", Method: zip.Deflate, Modified: time.Now()})
+	if err != nil {
+		ctx.ServerError("Failed to create zip file", err)
+		return
+	}
+
+	err = pprof.StartCPUProfile(f)
+	if err == nil {
+		time.Sleep(time.Duration(seconds) * time.Second)
+		pprof.StopCPUProfile()
+	} else {
+		_, _ = f.Write([]byte(err.Error()))
+	}
+
+	f, err = zipWriter.CreateHeader(&zip.FileHeader{Name: "goroutine-after.txt", Method: zip.Deflate, Modified: time.Now()})
+	if err != nil {
+		ctx.ServerError("Failed to create zip file", err)
+		return
+	}
+	_ = pprof.Lookup("goroutine").WriteTo(f, 1)
+}