Spans with distinct names get merged

If I do this, I get two spans:

```python
import distributed
from distributed.spans import span

client = distributed.Client()

def double(x):
return x * 2

with span("double-3"):
client.submit(double, 3)

# wait a bit

with span("double-5"):
client.submit(double, 5)
```

But if I don't wait a bit and just run this code all at once, I get a single span named `double-3`. That doesn't seem right to me.

I noticed this because I was trying to submit tasks in a loop so I could make them distinct spans, like so:

```python
futures = []
for i in range(10):
with span(f"double-{i}"):
f = client.submit(double, i)
futures.append(f)
```

But of course that fails in the same way: I get a single `double-0` span.

(I'm trying to figure out how to map a function over some data and have each of those tasks a distinct span.)