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_test.go

@@ -0,0 +1,93 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gtprof
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// "vendor span" is a simple demo for a span from a vendor library
+
+var vendorContextKey any = "vendorContextKey"
+
+type vendorSpan struct {
+	name     string
+	children []*vendorSpan
+}
+
+func vendorTraceStart(ctx context.Context, name string) (context.Context, *vendorSpan) {
+	span := &vendorSpan{name: name}
+	parentSpan, ok := ctx.Value(vendorContextKey).(*vendorSpan)
+	if ok {
+		parentSpan.children = append(parentSpan.children, span)
+	}
+	ctx = context.WithValue(ctx, vendorContextKey, span)
+	return ctx, span
+}
+
+// below "testTrace*" integrate the vendor span into our trace system
+
+type testTraceSpan struct {
+	vendorSpan *vendorSpan
+}
+
+func (t *testTraceSpan) addEvent(name string, cfg *EventConfig) {}
+
+func (t *testTraceSpan) recordError(err error, cfg *EventConfig) {}
+
+func (t *testTraceSpan) end() {}
+
+type testTraceStarter struct{}
+
+func (t *testTraceStarter) start(ctx context.Context, traceSpan *TraceSpan, internalSpanIdx int) (context.Context, traceSpanInternal) {
+	ctx, span := vendorTraceStart(ctx, traceSpan.name)
+	return ctx, &testTraceSpan{span}
+}
+
+func TestTraceStarter(t *testing.T) {
+	globalTraceStarters = []traceStarter{&testTraceStarter{}}
+
+	ctx := context.Background()
+	ctx, span := GetTracer().Start(ctx, "root")
+	defer span.End()
+
+	func(ctx context.Context) {
+		ctx, span := GetTracer().Start(ctx, "span1")
+		defer span.End()
+		func(ctx context.Context) {
+			_, span := GetTracer().Start(ctx, "spanA")
+			defer span.End()
+		}(ctx)
+		func(ctx context.Context) {
+			_, span := GetTracer().Start(ctx, "spanB")
+			defer span.End()
+		}(ctx)
+	}(ctx)
+
+	func(ctx context.Context) {
+		_, span := GetTracer().Start(ctx, "span2")
+		defer span.End()
+	}(ctx)
+
+	var spanFullNames []string
+	var collectSpanNames func(parentFullName string, s *vendorSpan)
+	collectSpanNames = func(parentFullName string, s *vendorSpan) {
+		fullName := parentFullName + "/" + s.name
+		spanFullNames = append(spanFullNames, fullName)
+		for _, c := range s.children {
+			collectSpanNames(fullName, c)
+		}
+	}
+	collectSpanNames("", span.internalSpans[0].(*testTraceSpan).vendorSpan)
+	assert.Equal(t, []string{
+		"/root",
+		"/root/span1",
+		"/root/span1/spanA",
+		"/root/span1/spanB",
+		"/root/span2",
+	}, spanFullNames)
+}