label information get lost after parquet serialization
Hi ! We did a release of `datasets` today that may fix this issue. Can you try updating `datasets` and trying again ?

EDIT: the issue is still there actually

I think we can fix that by storing the Features in the parquet schema metadata, and then reload them when loading the parquet file
## Describe the bug
In *dataset_info.json* file, information about the label get lost after the dataset serialization.

## Steps to reproduce the bug
```python
from datasets import load_dataset

# normal save
dataset = load_dataset('glue', 'sst2', split='train')
dataset.save_to_disk("normal_save")


# save after parquet serialization
dataset.to_parquet("glue-sst2-train.parquet")
dataset = load_dataset("parquet", data_files='glue-sst2-train.parquet')
dataset.save_to_disk("save_after_parquet")
```

## Expected results
I expected to keep label information in *dataset_info.json* file even after parquet serialization

## Actual results
In the normal serialization i got
```json
"label": {
      "num_classes": 2,
      "names": [
        "negative",
        "positive"
      ],
      "names_file": null,
      "id": null,
      "_type": "ClassLabel"
    },
```
And after parquet serialization i got
```json
"label": {
      "dtype": "int64",
      "id": null,
      "_type": "Value"
    },
```

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.18.0
- Platform: ubuntu 20.04
- Python version: 3.8.10
- PyArrow version: 6.0.1
