fix build performance tracing

[syphar]

I wanted to look into performance traces for builds, and discovered they are all over the place.

Mainly: the main docbuilder.build_package trace is not connected to the child traces.

I'm not 100% sure if these changes solve it completely, but I definitely discovered these issues, and they are fixed by the PR:

• we had logic in place that would force all builds to be traced, and only applies the sampling ratio to web requests. This didn't work, at least since the multi-crate migration. I validated what's in ctx.operation() and ctx.name() and fixed the filter.
• we have our custom spawn_blocking function that takes care of passing on the parent span into the task because tokio doesn't. I realized that block_on has the same issue. For now I just used the new fixed handle for builds. If it solves the issues there, I'll move the spawn_blocking fn into the handler too, and update the whole codebase.