feat: 🎸 add "truncated" field to /first-rows
fixes #1731.

It will require refreshing split-first-rows-from-streaming and split-first-rows-from-parquet for all the datasets. The backfill will do it since I updated the versions.

Also, I put the "truncated" field as optional for now, but once the update is complete, I'll create another PR to make it mandatory (we cannot do a database migration since we don't know the value).

---

Followup PRs:
- [x] increase version of `split-first-rows-from-streaming` - 350K waiting jobs now. Let's wait until Monday to do the following step -> https://github.com/huggingface/datasets-server/pull/1844
- [x] refresh the remaining cache entries until we get 0:
  ```
  db.cachedResponsesBlue.count({kind: {"$in": ["split-first-rows-from-streaming", "split-first-rows-from-parquet"]}, http_status: 200, "content.truncated": {"$exists": false}})
  ```

  List of datasets to refresh:

  ```
  db.cachedResponsesBlue.aggregate([
    {$match: {kind: "split-first-rows-from-parquet", http_status: 200, "content.truncated": {"$exists": false}}},
    {$group: {_id: "$dataset", count: {$sum: 1}}},
    {$sort: {count: -1}},
    {$project: {dataset: "$_id", _id: 0, count: 1}}
  ])
  ```
  and
  ```
  db.cachedResponsesBlue.aggregate([
    {$match: {kind: "split-first-rows-from-parquet", http_status: 200, "content.truncated": {"$exists": false}}},
    {$group: {_id: null, datasets: {$addToSet: "$dataset"}}},
  ])
  ```

  2023/09/25: refreshing the remaining cache entries for `split-first-rows-from-parquet`.
- [x] set `truncated` field as mandatory, not optional. https://github.com/huggingface/datasets-server/pull/1896
  
feat: 🎸 add "truncated" field to /first-rows: fixes #1731.

It will require refreshing split-first-rows-from-streaming and split-first-rows-from-parquet for all the datasets. The backfill will do it since I updated the versions.

Also, I put the "truncated" field as optional for now, but once the update is complete, I'll create another PR to make it mandatory (we cannot do a database migration since we don't know the value).

---

Followup PRs:
- [x] increase version of `split-first-rows-from-streaming` - 350K waiting jobs now. Let's wait until Monday to do the following step -> https://github.com/huggingface/datasets-server/pull/1844
- [x] refresh the remaining cache entries until we get 0:
  ```
  db.cachedResponsesBlue.count({kind: {"$in": ["split-first-rows-from-streaming", "split-first-rows-from-parquet"]}, http_status: 200, "content.truncated": {"$exists": false}})
  ```

  List of datasets to refresh:

  ```
  db.cachedResponsesBlue.aggregate([
    {$match: {kind: "split-first-rows-from-parquet", http_status: 200, "content.truncated": {"$exists": false}}},
    {$group: {_id: "$dataset", count: {$sum: 1}}},
    {$sort: {count: -1}},
    {$project: {dataset: "$_id", _id: 0, count: 1}}
  ])
  ```
  and
  ```
  db.cachedResponsesBlue.aggregate([
    {$match: {kind: "split-first-rows-from-parquet", http_status: 200, "content.truncated": {"$exists": false}}},
    {$group: {_id: null, datasets: {$addToSet: "$dataset"}}},
  ])
  ```

  2023/09/25: refreshing the remaining cache entries for `split-first-rows-from-parquet`.
- [x] set `truncated` field as mandatory, not optional. https://github.com/huggingface/datasets-server/pull/1896
  