Constrain type parameter of `RootSpanBuilder::on_request_end` with `actix_web::body::MessageBody`

[rharish101]

Is it possible to change the API to change the following in RootSpanBuilder:

fn on_request_end<B>(span: Span, outcome: &Result<ServiceResponse<B>, Error>);

to the following?:

fn on_request_end<B: MessageBody>(span: Span, outcome: &Result<ServiceResponse<B>, Error>);

This will allow using methods defined for MessageBody when creating custom RootSpanBuilders, such as MessageBody::size. The root span is always called on MessageBodys anyway, as seen here:

tracing-actix-web/src/middleware.rs

Lines 181 to 198 in c95a781

	impl<F, B, RootSpanType> Future for TracingResponse<F, RootSpanType>
	where
	F: Future<Output = Result<ServiceResponse<B>, Error>>,
	B: MessageBody + 'static,
	RootSpanType: RootSpanBuilder,
	{
	type Output = Result<ServiceResponse<StreamSpan<B>>, Error>;
	fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
	let this = self.project();
	let fut = this.fut;
	let span = this.span;
	span.in_scope(|| match fut.poll(cx) {
	Poll::Pending => Poll::Pending,
	Poll::Ready(outcome) => {
	RootSpanType::on_request_end(Span::current(), &outcome);

I can create a PR if this is fine.

[LukeMathWalker]

Yes, a PR would be welcome!

[rharish101]

I've created PR #93 to solve this.

[LukeMathWalker]

Merged and released as 0.7.0 - thanks!