Creating index-patterns causes search threadPool to be exhausted

[kkewwei]

Is your feature request related to a problem? Please describe.

1. when I set the ccs in the cluster settings(the address is pointed to itself to reproduce)

PUT _cluster/settings
{
  "persistent": {
      "cluster": {
         "remote": {
            "ccs1": {
               "mode": "proxy",
               "proxy_address": "127.0.0.1:9300"
            }
         }
      }
  }
}

and create a new index-patterns in the dashboard, I find that search thread pool is immediately exhausted in the target cluster: And search slow log is as follows:

POST *:*/_search
{
  "size": 0,
  "timeout": "30000ms",
  "query": {
    "match_all": {
      "boost": 1
    }
  },
  "track_total_hits": 2147483647,
  "aggregations": {
    "indices": {
      "terms": {
        "field": "_index",
        "size": 200,
        "min_doc_count": 1,
        "shard_min_doc_count": 0,
        "show_term_doc_count_error": false,
        "order": [
          {
            "_count": "desc"
          },
          {
            "_key": "asc"
          }
        ]
      }
    }
  }
}

2. As the total_shards=35720, which equals the total number shards of the cluster, we know that all the indices are aggregated by the query.

3. We know that the query is from the "Index_Patterns".

It seems that we send the query to get the indices list of target cluster, when the target cluster is very large, this query will become very dangerous.

Describe the solution you'd like If we should add the api in the opensearch code core to get indices list of the ccs target cluster?