Log slow API requests with server spans and client timing

internal/trace/trace.go

@@ -0,0 +1,71 @@
+package trace
+
+import (
+	"context"
+	"fmt"
+	"strings"
+	"sync"
+	"time"
+)
+
+type span struct {
+	name string
+	dur  time.Duration
+}
+
+type Trace struct {
+	mu    sync.Mutex
+	spans []span
+}
+
+type ctxKey struct{}
+
+func New(ctx context.Context) (context.Context, *Trace) {
+	t := &Trace{}
+	return context.WithValue(ctx, ctxKey{}, t), t
+}
+
+func Start(ctx context.Context, name string) func() {
+	start := time.Now()
+	return func() {
+		Add(ctx, name, time.Since(start))
+	}
+}
+
+func Add(ctx context.Context, name string, dur time.Duration) {
+	t, _ := ctx.Value(ctxKey{}).(*Trace)
+	if t == nil {
+		return
+	}
+	t.mu.Lock()
+	t.spans = append(t.spans, span{name: name, dur: dur})
+	t.mu.Unlock()
+}
+
+func (t *Trace) Header(total time.Duration) string {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	parts := []string{fmt.Sprintf("total;dur=%.1f", ms(total))}
+	for _, s := range t.spans {
+		parts = append(parts, fmt.Sprintf("%s;dur=%.1f", token(s.name), ms(s.dur)))
+	}
+	return strings.Join(parts, ", ")
+}
+
+func (t *Trace) String() string {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	parts := []string{}
+	for _, s := range t.spans {
+		parts = append(parts, fmt.Sprintf("%s %s", s.name, s.dur.Round(time.Millisecond)))
+	}
+	return strings.Join(parts, ", ")
+}
+
+func ms(d time.Duration) float64 {
+	return float64(d.Microseconds()) / 1000
+}
+
+func token(name string) string {
+	return strings.ReplaceAll(name, " ", "-")
+}