IO and Async Compute task spans

## What problem does this solve or what need does it fill?
Noticed that, when testing #12090, that Bevy does not have any spans for any of the tasks scheduled onto the IO and async compute task pols.

## What solution would you like?
Preferably all tasks scheduled onto the task pools should have some top level spans, perhaps by instrumenting all incoming tasks. If that's too involved or too high overhead, we should at the very minimum instrument all asset loads.

## What alternative(s) have you considered?
Leaving it be, shooting in the dark.