statistics are empty

I needed to know how many retries a function made.

I used [the example in the doc](https://tenacity.readthedocs.io/en/stable/#statistics) (v9.0.0):

```python
from tenacity import *

@retry(stop=stop_after_attempt(3))
def raise_my_exception():
raise Exception("Fail")

try:
raise_my_exception()
except Exception as e:
print(e)
pass

print(raise_my_exception.retry.statistics)
```

when executing:

```bash
RetryError[]
{}
```

statistics are empty

I was expecting

```python
{ "attempt_number" : 3 }
```