ensure current tracing span is passed into streaming html rewrite

[syphar]

Here I'm trying again to fix this.

Problem is: when we emit log-messages from inside the stream, or sentry errors, these don't contain any information from the parent spans. For example on this sentry we can't really see which crate is affected.

The initially obvious solution was #2874, where I use Span::enter and the guard object.

Some reading and testing made me understand where the memory leak was coming from. Generally the guard object coming from Span::enter should never be held across await-points. Which I did since I was holding onto it for the whole stream.

The solution is to use the async-capable alternative, which is annotating the future with .instrument(span). The new tracing-futures dependency is needed so we can directly annotate the stream.

Additionally I did forget to pass the span info into our render_in_threadpool clojures.

[GuillaumeGomez]

Feel free to merge once merge conflicts have been fixed.