type SingleHandler[Req, Resp RoundTripper] struct {
	id            HandlerID
	sharedResp    bool
	callReuseReq  bool
	ignoreNilConn bool

	newReq  func() Req
	newResp func() Resp

	recycleReq  func(Req)
	recycleResp func(Resp)
}

func recycleFunc[RT RoundTripper](newRT func() RT) (newFn func() RT, recycle func(r RT)) {
	rAny := any(newRT())
	var rZero RT
	if _, ok := rAny.(Recycler); ok {