3789: Improve the metrics r=dureuill a=irevoire

# Pull Request

## Related issue
Implements #3790
Associated specification: meilisearch/specifications#242

## Be cautious; it's DB-breaking 😱 

While reviewing and after merging this PR, be cautious; if you already have a `data.ms` and run meilisearch with this code on it, it won't work because we need to cache a new information on the index stats (that are backed up on disk). You'll get internal errors.

### About the breaking-change label

We only break the API of the metrics route, which does not pose any problem since it's experimental.

## What does this PR do?
- Create a method to get the « facet distribution » of the task queue.
- Prefix all the metrics by `meilisearch_`
- Add the real database size used by meilisearch
- Add metrics on the task queue
- Update the grafana dashboard to these new changes
- Move the dashboard to the `assets` directory
- Provide a new prometheus file to scrape meilisearch easily

Co-authored-by: Tamo <tamo@meilisearch.com>

* Bump openapi.yaml file

* Facet Search API (#246)

* Init specification

* Adjust spec files to PR number

* Fix content-type open-api

* Remove fix content-type

* Add the telemetry

* Update text/0246-facet-search-api.md

Co-authored-by: cvermand <33010418+bidoubiwa@users.noreply.github.com>

* Update text/0034-telemetry-policies.md

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Co-authored-by: Clément Renault <clement@meilisearch.com>
Co-authored-by: cvermand <33010418+bidoubiwa@users.noreply.github.com>

* Faceting Setting API - sortFacetValuesBy  (#247)

* init spec

* Add property description and open-api.yaml

* Removes future possibility

* Update the telemetry

* Update the error of the sortFacetValuesBy setting

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Co-authored-by: Clément Renault <clement@meilisearch.com>

* Vector Search - `EXPERIMENTAL` (#248)

* Init spec

* Fix the vector store fields

* Add an information on the invalid_search_vector error code

* Add an information on the invalid_vectors_field error codes

* Define the new max_vector_size analytic

* Update the open-api file with vector capabilities

* Apply suggestions from code review

* Update open-api.yaml

Co-authored-by: Maria Craig <marycraig90@gmail.com>

* Update open-api.yaml

Co-authored-by: Maria Craig <marycraig90@gmail.com>

* Update text/0118-search-api.md

Co-authored-by: Maria Craig <marycraig90@gmail.com>

* Update text/0061-error-format-and-definitions.md

Co-authored-by: Maria Craig <marycraig90@gmail.com>

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Co-authored-by: Kerollmops <clement@meilisearch.com>
Co-authored-by: Maria Craig <marycraig90@gmail.com>

* Tasks route total (#253)

* Update the tasks route info to add the total field

* Update the OpenApi of the tasks route

* Document the latest metrics improvements (#242)

* document the latest metrics improvements

* Update text/0174-metrics-api.md

Co-authored-by: Guillaume Mourier <guillaume@meilisearch.com>

* Update text/0174-metrics-api.md

Co-authored-by: Guillaume Mourier <guillaume@meilisearch.com>

* adds some details on the meilisearch_db_size metrics

* update the open api metrics smaple

* update a bad copy paste

* Update open-api.yaml

* prefix prometheus metrics by meilisearch_

* Introduce two new metrics

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Co-authored-by: Guillaume Mourier <guillaume@meilisearch.com>
Co-authored-by: Kerollmops <clement@meilisearch.com>

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Co-authored-by: Guillaume Mourier <guillaume@meilisearch.com>
Co-authored-by: Clément Renault <clement@meilisearch.com>
Co-authored-by: cvermand <33010418+bidoubiwa@users.noreply.github.com>
Co-authored-by: Clément Renault <renault.cle@gmail.com>
Co-authored-by: Tamo <tamo@meilisearch.com>