**Elasticsearch version** (`bin/elasticsearch --version`): v7+

**Description of the problem including expected versus actual behavior**:

When used with an alias that has a filter, only the foreground set is filtered. The background set ignores the alias filter. This skews the results, especially when the user supplies an additional foreground filter, assuming the background filter will respect the alias filter.

**Steps to reproduce**:

```
PUT sig_terms_bug
{
  "mappings": {
    "properties": {
      "field1": {
        "type": "keyword"
      },
      "field2": {
        "type": "keyword"
      },
      "field3": {
        "type": "keyword"
      }
    }
  }
}

POST sig_terms_bug/_bulk?refresh=true
{ "index" : {} }
{ "field1" : "failed", "field2": "frontend-node", "field3": "foo" }
{ "index" : {} }
{ "field1" : "failed", "field2": "frontend-node", "field3": "bar" }
{ "index" : {} }
{ "field1" : "failed", "field2": "frontend-node", "field3": "bar" }
{ "index" : {} }
{ "field1" : "success", "field2": "frontend-node", "field3": "bar" }
{ "index" : {} }
{ "field1" : "success", "field2": "frontend-node", "field3": "bar" }
{ "index" : {} }
{ "field1" : "failed", "field2": "backend-node", "field3": "foo" }
{ "index" : {} }
{ "field1" : "failed", "field2": "backend-node", "field3": "bar" }
{ "index" : {} }
{ "field1" : "failed", "field2": "backend-node", "field3": "bar" }
{ "index" : {} }
{ "field1" : "success", "field2": "backend-node", "field3": "bar" }
{ "index" : {} }
{ "field1" : "success", "field2": "backend-node", "field3": "bar" }
{ "index" : {} }

POST _aliases
{
  "actions": [
    {
      "add": {
        "index": "sig_terms_bug",
        "alias": "sig_terms_bug_alias",
        "filter": {
          "bool": {
            "filter": [
              {
                "term": {
                  "field2": "frontend-node"
                }
              }
            ]
          }
        }
      }
    }
  ]
}

GET sig_terms_bug_alias/_search?track_total_hits=true&size=0&filter_path=aggregations
{
  "aggs": {
    "sig_term": {
      "significant_terms": {
        "field": "field3"
      }
    }
  }
}
```

This results in:
```
{
  "aggregations" : {
    "sig_term" : {
      "doc_count" : 5,
      "bg_count" : 10,
      "buckets" : [ ]
    }
  }
}
```

It seems to me that the `bg_count` should respect the alias filter.

server/src/main/java/org/elasticsearch/search/aggregations/bucket/terms/SignificantTermsAggregationBuilder.java
throw new IllegalArgumentException("[shardSize] must be greater than  0. Found [" + shardSize + "] in [" + name + "]");
        }
        bucketCountThresholds.setShardSize(shardSize);
        return this;
    }
    /**
     * Set the minimum document count terms should have in order to appear in
     * the response.
     */
    public SignificantTermsAggregationBuilder minDocCount(long minDocCount) {
        if (minDocCount < 0) {
            throw new IllegalArgumentException(
                "[minDocCount] must be greater than or equal to 0. Found [" + minDocCount + "] in [" + name + "]"
            );
        }
        bucketCountThresholds.setMinDocCount(minDocCount);
        return this;
    }
    /**
     * Set the minimum document count terms should have on the shard in order to
     * appear in the response.
     */
    public SignificantTermsAggregationBuilder shardMinDocCount(long shardMinDocCount) {
        if (shardMinDocCount < 0) {
            throw new IllegalArgumentException(
                "[shardMinDocCount] must be greater than or equal to 0. Found [" + shardMinDocCount + "] in [" + name + "]"
            );
        }
        bucketCountThresholds.setShardMinDocCount(shardMinDocCount);
        return this;