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/stacktrace.go

@@ -0,0 +1,48 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package admin
+
+import (
+	"net/http"
+	"runtime"
+
+	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/process"
+	"code.gitea.io/gitea/modules/setting"
+)
+
+// Stacktrace show admin monitor goroutines page
+func Stacktrace(ctx *context.Context) {
+	ctx.Data["Title"] = ctx.Tr("admin.monitor")
+	ctx.Data["PageIsAdminMonitorStacktrace"] = true
+
+	ctx.Data["GoroutineCount"] = runtime.NumGoroutine()
+
+	show := ctx.FormString("show")
+	ctx.Data["ShowGoroutineList"] = show
+	// by default, do not do anything which might cause server errors, to avoid unnecessary 500 pages.
+	// this page is the entrance of the chance to collect diagnosis report.
+	if show != "" {
+		showNoSystem := show == "process"
+		processStacks, processCount, _, err := process.GetManager().ProcessStacktraces(false, showNoSystem)
+		if err != nil {
+			ctx.ServerError("GoroutineStacktrace", err)
+			return
+		}
+
+		ctx.Data["ProcessStacks"] = processStacks
+		ctx.Data["ProcessCount"] = processCount
+	}
+
+	ctx.HTML(http.StatusOK, tplStacktrace)
+}
+
+// StacktraceCancel cancels a process
+func StacktraceCancel(ctx *context.Context) {
+	pid := ctx.Params("pid")
+	process.GetManager().Cancel(process.IDType(pid))
+	ctx.JSON(http.StatusOK, map[string]interface{}{
+		"redirect": setting.AppSubURL + "/admin/monitor/stacktrace",
+	})
+}