**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
}
        }
    }
    public void testSomeDocsWithoutStringFieldviaGlobalOrds() throws IOException {
        testSomeDocsWithoutStringField("global_ordinals");
    }
    public void testSomeDocsWithoutStringFieldViaMap() throws IOException {
        testSomeDocsWithoutStringField("map");
    }
    /**
     * Make sure that when the field a segment doesn't contain the field we
     * still include the count of its matching documents
     * in {@link InternalMappedSignificantTerms#getSubsetSize()}.
     */
    private void testSomeDocsWithoutStringField(String executionHint) throws IOException {
        try (Directory dir = newDirectory()) {
            try (RandomIndexWriter writer = new RandomIndexWriter(random(), dir)) {
                Document d = new Document();
                d.add(new SortedDocValuesField("f", new BytesRef("f")));
                writer.addDocument(d);
                writer.flush();
                writer.addDocument(new Document());
                try (IndexReader reader = maybeWrapReaderEs(writer.getReader())) {
                    IndexSearcher searcher = newIndexSearcher(reader);
                    SignificantTermsAggregationBuilder request = new SignificantTermsAggregationBuilder("f").field("f")
                        .executionHint(executionHint);
                    SignificantStringTerms result = searchAndReduce(searcher, new MatchAllDocsQuery(), request, keywordField("f"));
                    assertThat(result.getSubsetSize(), equalTo(2L));
                }
            }