[8.14](backport #39165) [Eleasticsearch] Remove hidden field from indice metricset docs

[mergify]

Proposed commit message

This PR removes the hidden flag introduced by #18639. The SM UI doesn't use this flag, and retrieving which indices are hidden is an expensive operation that can sometimes cause problems such as timeouts depending on how large a cluster is.

elasticsearch": {
  "cluster": {
    "id": "5QYQ1YeeRjy9MJtahqzY6w",
    "name": "elasticsearch"
  },
  "index": {
    "status": "green",
    "hidden": false,
    "shards": {
      "total": 1,
      "primaries": 1
    }

   ...
  }
  ...
}

"elasticsearch": {
  "cluster": {
    "id": "5QYQ1YeeRjy9MJtahqzY6w",
    "name": "elasticsearch"
  },
  "index": {
    "status": "green",
    "shards": {
      "total": 1,
      "primaries": 1
    }
   ...
  }
 ...
}

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

• Start a local ES and kibana instances
• Start metricbeat with elasticsearch module enabled
• Run the query below and check if there is no hidden field in the index object

GET .monitoring-es-*,metrics-*/_search
{
  "size": 5,
  "query": {
    "bool": {
      "filter": [
        {
          "bool": {
            "should": [
              {
                "term": {
                  "data_stream.dataset": "elasticsearch.stack_monitoring.index"
                }
              },
              {
                "term": {
                  "metricset.name": "index"
                }
              },
              {
                "term": {
                  "type": "index_stats"
                }
              }
            ]
          }
        },
        {
          "range": {
            "timestamp": {
              "gte": "now-15m",
              "lte": "now"
            }
          }
        }
      ]
    }
  },
  "collapse": {
    "field": "index_stats.index",
    "inner_hits": {
      "name": "earliest",
      "size": 1,
      "sort": [
        {
          "timestamp": {
            "order": "asc",
            "unmapped_type": "long"
          }
        }
      ]
    }
  },
  "sort": [
    {
      "timestamp": {
        "order": "desc",
        "unmapped_type": "long"
      }
    }
  ]
}

Related issues

closes #39161

---

This is an automatic backport of pull request #39165 done by [Mergify](https://mergify.com).

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[mergify]

This pull request has not been merged yet. Could you please review and merge it @crespocarlos? 🙏