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>

modules/gtprof/trace_const.go

@@ -0,0 +1,19 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gtprof
+
+// Some interesting names could be found in https://github.com/open-telemetry/opentelemetry-go/tree/main/semconv
+
+const (
+	TraceSpanHTTP     = "http"
+	TraceSpanGitRun   = "git-run"
+	TraceSpanDatabase = "database"
+)
+
+const (
+	TraceAttrFuncCaller = "func.caller"
+	TraceAttrDbSQL      = "db.sql"
+	TraceAttrGitCommand = "git.command"
+	TraceAttrHTTPRoute  = "http.route"
+)