Skip reindexing when modifying unknown faceted fields

[Kerollmops]

This PR improves Meilisearch's decision to reindex when a faceted field is added to the settings, but not a single document contains this field. It is effectively a waste of time to reindex documents when the engine needs to know a field.

This is related to a conversation we have with our biggest customer (internal link). They have 170 million documents, so reindexing this amount would be problematic.

---

The image is available by using the following Docker command. You can see the advancement of the image's build on the GitHub CI page.

docker pull getmeili/meilisearch:prototype-no-reindex-unknown-fields-0

Here is the hand-made test that shows that when modifying unknown filterable attributes, here lol, it doesn't reindex. However, when modifying the known genre field, it does reindex. You can see all that by looking at the time spent processing the update.

{
  "uid": 3,
  "indexUid": "movies",
  "status": "succeeded",
  "type": "settingsUpdate",
  "canceledBy": null,
  "details": {
    "filterableAttributes": [
      "genres"
    ]
  },
  "error": null,
  "duration": "PT9.237703S",
  "enqueuedAt": "2024-03-12T15:34:26.836083Z",
  "startedAt": "2024-03-12T15:34:26.836374Z",
  "finishedAt": "2024-03-12T15:34:36.074077Z"
},
{
  "uid": 2,
  "indexUid": "movies",
  "status": "succeeded",
  "type": "settingsUpdate",
  "canceledBy": null,
  "details": {
    "filterableAttributes": [
      "lol"
    ]
  },
  "error": null,
  "duration": "PT0.000751S",
  "enqueuedAt": "2024-03-12T15:33:53.563923Z",
  "startedAt": "2024-03-12T15:33:53.565259Z",
  "finishedAt": "2024-03-12T15:33:53.56601Z"
},
{
  "uid": 0,
  "indexUid": "movies",
  "status": "succeeded",
  "type": "documentAdditionOrUpdate",
  "canceledBy": null,
  "details": {
    "receivedDocuments": 31944,
    "indexedDocuments": 31944
  },
  "error": null,
  "duration": "PT3.120723S",
  "enqueuedAt": "2024-02-17T10:35:55.042864Z",
  "startedAt": "2024-02-17T10:35:55.043505Z",
  "finishedAt": "2024-02-17T10:35:58.164228Z"
}

[dureuill]

bors merge

[meili-bors]

Build succeeded:

• Run Clippy
• Run Rustfmt
• Run tests in debug
• Tests on macos-12
• Tests on ubuntu-18.04
• Tests on windows-2022