Can't serialize functions decorated with `@span`

This gives me an error:

```python

import distributed
from distributed.spans import span

client = distributed.Client()

@span("my-double")
def double(x):
return x * 2

X = client.map(double, range(10))
```

I get

```
TypeError: ('Could not serialize object of type HighLevelGraph', '\n 0. 6029444544\n>')
```

This is with dask and distributed `2023.6.2a230627`, installed from `dask/label/dev`, as well as `2023.6.1` and `2023.6.0`.

Am I supposed to be able to do this? Or am I doing something wrong? The same code minus the `@span` decorator works.