Log slow API requests with server spans and client timing

internal/server/timing.go

@@ -0,0 +1,50 @@
+package server
+
+import (
+	"log/slog"
+	"net/http"
+	"time"
+
+	"g.fc.run/ai/byre/internal/trace"
+)
+
+const slowRequest = 200 * time.Millisecond
+
+type timedWriter struct {
+	http.ResponseWriter
+	start  time.Time
+	trace  *trace.Trace
+	status int
+}
+
+func (w *timedWriter) WriteHeader(status int) {
+	if w.status == 0 {
+		w.status = status
+		w.Header().Set("Server-Timing", w.trace.Header(time.Since(w.start)))
+	}
+	w.ResponseWriter.WriteHeader(status)
+}
+
+func (w *timedWriter) Write(b []byte) (int, error) {
+	if w.status == 0 {
+		w.WriteHeader(http.StatusOK)
+	}
+	return w.ResponseWriter.Write(b)
+}
+
+func (w *timedWriter) Unwrap() http.ResponseWriter {
+	return w.ResponseWriter
+}
+
+func timing(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		ctx, t := trace.New(r.Context())
+		tw := &timedWriter{ResponseWriter: w, start: time.Now(), trace: t}
+		next.ServeHTTP(tw, r.WithContext(ctx))
+		took := time.Since(tw.start)
+		if took < slowRequest || tw.status == http.StatusSwitchingProtocols || tw.Header().Get("Content-Type") == "text/event-stream" {
+			return
+		}
+		slog.Info("request slow", "method", r.Method, "path", r.URL.Path, "status", tw.status, "took", took, "spans", t.String())
+	})
+}