feat: add PprofLabeling wrapper labeling each request via runtime/pprof.Do - #46

Adds a PprofLabeling wrapper that serves every request inside runtime/pprof.Do,
so CPU profile samples and goroutine tracebacks can be attributed to their route.

• Labels default to the matched r.Pattern; a Labels function can add pairs like a trace ID
• Registered via Use, not bundled in Default()
• Adds tests and a runnable example under examples/pprof-labeling

Usage

pl := &sim.PprofLabeling{Labels: func(r *http.Request) []string {
    return []string{"pattern", r.Pattern, "trace_id", traceID(r.Context())}
}}

app := sim.NewApp()
app.Use(traceID.Handler, pl.Handler, new(sim.Recovery).Handler)

Labels default to the matched route pattern when Labels is nil. Filter samples while profiling:

go tool pprof -tagfocus='pattern=GET /users/{id}' http://host/debug/pprof/profile