Support performance trace (#32973)

1. Add a OpenTelemetry-like shim-layer to collect traces
2. Add a simple builtin trace collector and exporter, end users could
download the diagnosis report to get the traces.

This PR's design is quite lightweight, no hard-dependency, and it is
easy to improve or remove. We can try it on gitea.com first to see
whether it works well, and fine tune the details.

---------

Co-authored-by: silverwind <me@silverwind.io>

routers/web/admin/perftrace.go

@@ -0,0 +1,18 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package admin
+
+import (
+	"net/http"
+
+	"code.gitea.io/gitea/modules/tailmsg"
+	"code.gitea.io/gitea/services/context"
+)
+
+func PerfTrace(ctx *context.Context) {
+	monitorTraceCommon(ctx)
+	ctx.Data["PageIsAdminMonitorPerfTrace"] = true
+	ctx.Data["PerfTraceRecords"] = tailmsg.GetManager().GetTraceRecorder().GetRecords()
+	ctx.HTML(http.StatusOK, tplPerfTrace)
+}