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.go

@@ -0,0 +1,175 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package gtprof
+
+import (
+	"context"
+	"fmt"
+	"sync"
+	"time"
+
+	"code.gitea.io/gitea/modules/util"
+)
+
+type contextKey struct {
+	name string
+}
+
+var contextKeySpan = &contextKey{"span"}
+
+type traceStarter interface {
+	start(ctx context.Context, traceSpan *TraceSpan, internalSpanIdx int) (context.Context, traceSpanInternal)
+}
+
+type traceSpanInternal interface {
+	addEvent(name string, cfg *EventConfig)
+	recordError(err error, cfg *EventConfig)
+	end()
+}
+
+type TraceSpan struct {
+	// immutable
+	parent           *TraceSpan
+	internalSpans    []traceSpanInternal
+	internalContexts []context.Context
+
+	// mutable, must be protected by mutex
+	mu         sync.RWMutex
+	name       string
+	statusCode uint32
+	statusDesc string
+	startTime  time.Time
+	endTime    time.Time
+	attributes []*TraceAttribute
+	children   []*TraceSpan
+}
+
+type TraceAttribute struct {
+	Key   string
+	Value TraceValue
+}
+
+type TraceValue struct {
+	v any
+}
+
+func (t *TraceValue) AsString() string {
+	return fmt.Sprint(t.v)
+}
+
+func (t *TraceValue) AsInt64() int64 {
+	v, _ := util.ToInt64(t.v)
+	return v
+}
+
+func (t *TraceValue) AsFloat64() float64 {
+	v, _ := util.ToFloat64(t.v)
+	return v
+}
+
+var globalTraceStarters []traceStarter
+
+type Tracer struct {
+	starters []traceStarter
+}
+
+func (s *TraceSpan) SetName(name string) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.name = name
+}
+
+func (s *TraceSpan) SetStatus(code uint32, desc string) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.statusCode, s.statusDesc = code, desc
+}
+
+func (s *TraceSpan) AddEvent(name string, options ...EventOption) {
+	cfg := eventConfigFromOptions(options...)
+	for _, tsp := range s.internalSpans {
+		tsp.addEvent(name, cfg)
+	}
+}
+
+func (s *TraceSpan) RecordError(err error, options ...EventOption) {
+	cfg := eventConfigFromOptions(options...)
+	for _, tsp := range s.internalSpans {
+		tsp.recordError(err, cfg)
+	}
+}
+
+func (s *TraceSpan) SetAttributeString(key, value string) *TraceSpan {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	s.attributes = append(s.attributes, &TraceAttribute{Key: key, Value: TraceValue{v: value}})
+	return s
+}
+
+func (t *Tracer) Start(ctx context.Context, spanName string) (context.Context, *TraceSpan) {
+	starters := t.starters
+	if starters == nil {
+		starters = globalTraceStarters
+	}
+	ts := &TraceSpan{name: spanName, startTime: time.Now()}
+	parentSpan := GetContextSpan(ctx)
+	if parentSpan != nil {
+		parentSpan.mu.Lock()
+		parentSpan.children = append(parentSpan.children, ts)
+		parentSpan.mu.Unlock()
+		ts.parent = parentSpan
+	}
+
+	parentCtx := ctx
+	for internalSpanIdx, tsp := range starters {
+		var internalSpan traceSpanInternal
+		if parentSpan != nil {
+			parentCtx = parentSpan.internalContexts[internalSpanIdx]
+		}
+		ctx, internalSpan = tsp.start(parentCtx, ts, internalSpanIdx)
+		ts.internalContexts = append(ts.internalContexts, ctx)
+		ts.internalSpans = append(ts.internalSpans, internalSpan)
+	}
+	ctx = context.WithValue(ctx, contextKeySpan, ts)
+	return ctx, ts
+}
+
+type mutableContext interface {
+	context.Context
+	SetContextValue(key, value any)
+	GetContextValue(key any) any
+}
+
+// StartInContext starts a trace span in Gitea's mutable context (usually the web request context).
+// Due to the design limitation of Gitea's web framework, it can't use `context.WithValue` to bind a new span into a new context.
+// So here we use our "reqctx" framework to achieve the same result: web request context could always see the latest "span".
+func (t *Tracer) StartInContext(ctx mutableContext, spanName string) (*TraceSpan, func()) {
+	curTraceSpan := GetContextSpan(ctx)
+	_, newTraceSpan := GetTracer().Start(ctx, spanName)
+	ctx.SetContextValue(contextKeySpan, newTraceSpan)
+	return newTraceSpan, func() {
+		newTraceSpan.End()
+		ctx.SetContextValue(contextKeySpan, curTraceSpan)
+	}
+}
+
+func (s *TraceSpan) End() {
+	s.mu.Lock()
+	s.endTime = time.Now()
+	s.mu.Unlock()
+
+	for _, tsp := range s.internalSpans {
+		tsp.end()
+	}
+}
+
+func GetTracer() *Tracer {
+	return &Tracer{}
+}
+
+func GetContextSpan(ctx context.Context) *TraceSpan {
+	ts, _ := ctx.Value(contextKeySpan).(*TraceSpan)
+	return ts
+}