[BUG] doc_count_error_upper_bound for terms agg should not show 0 when descending _count sort is not used

[jed326]

Describe the bug

Encountered this as a part of #11732

doc_count_error is only relevant when the buckets are sorted in descending _count order (I think this is a documentation gap too, but that's a separate issue https://opensearch.org/docs/latest/aggregations/bucket/terms/). If a different sort order is used then doc_count_error_upper_bound currently shows 0. This can be confusing for users since it implies that there are no candidate buckets being eliminated by shard_size.

Related component

Search:Aggregations

To Reproduce

Example using noaa data from OSB but the underlying data doesn't really matter:

GET weather-data-2016-multi/_search
{
  "size": 0,
  "aggs": {
    "station": {
      "terms": {
        "field": "station.elevation",
        "size": 1
        , "order": {
          "_key": "asc"
        }
      }
    }
  }
}

{
  "took": 1256,
  "timed_out": false,
  "terminated_early": true,
  "_shards": {
    "total": 2,
    "successful": 2,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": {
      "value": 10000,
      "relation": "gte"
    },
    "max_score": null,
    "hits": []
  },
  "aggregations": {
    "station": {
      "doc_count_error_upper_bound": 0,
      "sum_other_doc_count": 33581715,
      "buckets": [
        {
          "key": -999.9000244140625,
          "doc_count": 77766
        }
      ]
    }
  }
}

Expected behavior

Whenever the doc_count_error cannot be calculated, it either show not be returned in the response body or it should return -1 to differentiate the cases where there is no error vs cases where it cannot be computed.

Additional Details

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• OS: [e.g. iOS]
• Version [e.g. 22]

Additional context
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[peternied]

[Triage - attendees 1 2]
@jed326 Thanks for filing