Add SparseVectorStats

[kderusso]

Relates to #98275

Adds statistics on the number of sparse_vector fields in an index or cluster.

Because we can't pull dimensionality from the Lucene index, this relies on sparse_vector mappings to identify the documents to calculate fields for.

Here is an example script to test:

PUT my-index-1
{
  "mappings": {
    "properties": {
      "sparse_field1": {
        "type": "sparse_vector"
      },
      "sparse_field2": {
        "type": "sparse_vector"
      },
      "dense_field": {
        "type": "dense_vector"
      },
      "nonsparse_field": {
        "type": "keyword"
      }
    }
  },
  "settings": {
    "index.number_of_shards": 1,
    "index.number_of_replicas": 1
  }
}

// my-index-2 has 3 sparse_vector fields
PUT my-index-2
{
  "mappings": {
    "properties": {
      "sparse_field1": {
        "type": "sparse_vector"
      },
      "sparse_field2": {
        "type": "sparse_vector"
      },
      "sparse_field3": {
        "type": "sparse_vector"
      },
      "nonsparse_field": {
        "type": "keyword"
      }
    }
  },
  "settings": {
    "index.number_of_shards": 1,
    "index.number_of_replicas": 1
  }
}

PUT my-index-1/_doc/1
{
  "sparse_field1": { "a": 1, "b": 2 },
  "sparse_field2": { "c": 3, "d": 4, "e": 5},
  "dense_field": [1, 2, 3]
}

PUT my-index-1/_doc/2
{
  "sparse_field1": { "f": 1, "g": 2, "h": 3, "i": 4},
  "sparse_field2": { "j": 5, "k": 6, "l": 7},
  "dense_field": [2, 3, 4]
}

PUT my-index-2/_doc/1
{
  "sparse_field1": { "m": 1, "n": 2 },
  "sparse_field2": { "o": 3, "p": 4, "q": 5},
  "sparse_field3": { "a": 1, "b": 2 },
  "nonsparse_field": "cupcakes"
}

PUT my-index-2/_doc/2
{
  "nonsparse_field": "eclairs"
}

POST my-index-1/_refresh

POST my-index-2/_refresh

// _all returns 7 sparse_vector fields, which are then broken down by index
GET _stats/sparse_vector

// correctly returns 4 sparse_vector fields
GET my-index-1/_stats/sparse_vector

// correctly returns 3 sparse_vector fields
GET my-index-2/_stats/sparse_vector

GET /_nodes/stats

GET /_cat/shards?h=i,dvc,svc

[github-actions]

Documentation preview:

• ✨ Changed pages

[carlosdelest]

I think the docs refactoring could belong to a separate PR, labeled with "docs", so it's easier to review

[kderusso]

Note: @jimczi this does not work, so I removed it from the documentation. It can be added back if we decide to fix support for _cat/nodes.