if rl := respLoggerFromRequest(req); rl != nil {
		return rl.w
	}
	return w
}

// StacktraceWhen sets the stacktrace logging predicate, which decides when to log a stacktrace.
// There's a default, so you don't need to call this unless you don't like the default.
func (rl *respLogger) StacktraceWhen(pred StacktracePred) *respLogger {
	rl.mutex.Lock()