ensure current tracing span is passed into streaming html rewrite

[syphar]

I want to

• see the parent spans from the request when logging errors while rewriting
• have a separate span for performance tracing the rewriter.

Coming from these errors, where I want to see if they are flakes.

I'm also thinking of adding spans to the tokio-task & the render-in-threadpool call, but in there we don't log anything.