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](11441346/gitea-diagnosis-20230510-192913.zip)
, use "go tool pprof profile.dat" to view the report.


Screenshots:


![image](320659b4-2eda-4def-8dc0-5ea08d578063)


![image](c5c46fae-9dc0-44ca-8cd3-57beedc5035e)


![image](6168a811-42a1-4e64-a263-0617a6c8c4fe)

---------

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

routers/web/admin/queue_tester.go

@@ -0,0 +1,72 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package admin
+
+import (
+	gocontext "context"
+	"sync"
+	"time"
+
+	"code.gitea.io/gitea/modules/graceful"
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/queue"
+	"code.gitea.io/gitea/modules/setting"
+)
+
+var testQueueOnce sync.Once
+
+// initTestQueueOnce initializes the test queue for dev mode
+// the test queue will also be shown in the queue list
+// developers could see the queue length / worker number / items number on the admin page and try to remove the items
+func initTestQueueOnce() {
+	testQueueOnce.Do(func() {
+		qs := setting.QueueSettings{
+			Name:        "test-queue",
+			Type:        "channel",
+			Length:      20,
+			BatchLength: 2,
+			MaxWorkers:  3,
+		}
+		testQueue, err := queue.NewWorkerPoolQueueBySetting("test-queue", qs, func(t ...int64) (unhandled []int64) {
+			for range t {
+				select {
+				case <-graceful.GetManager().ShutdownContext().Done():
+				case <-time.After(5 * time.Second):
+				}
+			}
+			return nil
+		}, true)
+		if err != nil {
+			log.Error("unable to create test queue: %v", err)
+			return
+		}
+
+		queue.GetManager().AddManagedQueue(testQueue)
+		testQueue.SetWorkerMaxNumber(5)
+		go graceful.GetManager().RunWithShutdownFns(testQueue.Run)
+		go graceful.GetManager().RunWithShutdownContext(func(ctx gocontext.Context) {
+			cnt := int64(0)
+			adding := true
+			for {
+				select {
+				case <-ctx.Done():
+				case <-time.After(500 * time.Millisecond):
+					if adding {
+						if testQueue.GetQueueItemNumber() == qs.Length {
+							adding = false
+						}
+					} else {
+						if testQueue.GetQueueItemNumber() == 0 {
+							adding = true
+						}
+					}
+					if adding {
+						_ = testQueue.Push(cnt)
+						cnt++
+					}
+				}
+			}
+		})
+	})
+}