Log fork timings (#4618)
This includes a functional change, we now skip the forked state pop/push
if we didn't fork.
From transformers:
```
DEBUG Pre-fork split universal took 0.036s
DEBUG Split python_version >= '3.10' and python_version >= '3.10' and platform_system == 'Darwin' and python_version >= '3.11' and python_version >= '3.12' and python_version >= '3.6' and platform_system == 'Linux' and platform_machine == 'aarch64' took 0.048s
DEBUG Split python_version <= '3.9' and platform_system == 'Darwin' and platform_machine == 'arm64' and python_version >= '3.7' and python_version >= '3.8' and python_version >= '3.9' took 0.038s
```
The messages could use simplification from
https://github.com/astral-sh/uv/issues/4536
We can consider nested spans in the future but this works nicely for
now.
