**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/test/java/org/elasticsearch/search/aggregations/bucket/terms/SignificantTermsAggregatorTests.java
try (Directory dir = newDirectory()) {
            try (RandomIndexWriter writer = new RandomIndexWriter(random(), dir)) {
                for (int i = 0; i < 10; i++) {
                    for (int j = 0; j < 10; j++) {
                        for (int k = 0; k < 10; k++) {
                            Document d = new Document();
                            d.add(new SortedDocValuesField("i", new BytesRef(Integer.toString(i))));
                            d.add(new SortedDocValuesField("j", new BytesRef(Integer.toString(j))));
                            d.add(new SortedDocValuesField("k", new BytesRef(Integer.toString(k))));
                            writer.addDocument(d);
                        }
                    }
                }
                try (IndexReader reader = maybeWrapReaderEs(writer.getReader())) {
                    IndexSearcher searcher = newIndexSearcher(reader);
                    SignificantTermsAggregationBuilder kRequest = new SignificantTermsAggregationBuilder("k").field("k")
                        .minDocCount(0)
                        .executionHint(executionHint);
                    SignificantTermsAggregationBuilder jRequest = new SignificantTermsAggregationBuilder("j").field("j")
                        .minDocCount(0)
                        .executionHint(executionHint)
                        .subAggregation(kRequest);
                    SignificantTermsAggregationBuilder request = new SignificantTermsAggregationBuilder("i").field("i")
                        .minDocCount(0)
                        .executionHint(executionHint)
                        .subAggregation(jRequest);
                    SignificantStringTerms result = searchAndReduce(
                        searcher,
                        new MatchAllDocsQuery(),
                        request,