Delete InstrumentedAssetLoader and InstrumentedAssetProcessor, but keep the instrumentation.

[andriyDev]

Objective

• In Instrument asset loading and processing. #12988, we added tracing spans to asset loading and processing, by creating wrapper structs InstrumentedAssetLoader and InstrumentedAssetProcessor.
• Unfortunately, this causes an issue where ErasedAssetLoader::{default_meta, type_path} and ErasedProcessor::default_meta can return the wrong type path when using the trace feature, and this type path could be written into a meta file, which will be rendered unloadable/unprocessable.
  • Note even though the loader/processor's type use to have the instrumentation wrapper, when registering the loader/processor, we would actually register the loader/processor under the original type names. So you could still load non-trace written meta files, but trace written meta files would fail to load whether trace was enabled or not!

Solution

• Remove the wrapper loader/processor and instead just create the spans at the callsites. I checked and there's a single callsite of load and a single callsite of process - very good!
• I also added testing that was omitted in Stop automatically generating meta files for assets while using asset processing. #17216 (due to missing test utilities which have since been added). This is how I found out about this problem! Score one for testing.

Testing

• We still produce spans in the asset_processing example for both loading and processing.

Note: We don't need a migration guide since users should really not be using these directly, and any meta files using these would just be broken.

[kfc35]

I appreciate fun test text!!
(Logic looks good)