**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
keywordField("i"),
                        keywordField("j"),
                        keywordField("k")
                    );
                    assertThat(result.getSubsetSize(), equalTo(1000L));
                    for (int i = 0; i < 10; i++) {
                        SignificantStringTerms.Bucket iBucket = result.getBucketByKey(Integer.toString(i));
                        assertThat(iBucket.getDocCount(), equalTo(100L));
                        SignificantStringTerms jAgg = iBucket.getAggregations().get("j");
                        assertThat(jAgg.getSubsetSize(), equalTo(100L));
                        for (int j = 0; j < 10; j++) {
                            SignificantStringTerms.Bucket jBucket = jAgg.getBucketByKey(Integer.toString(j));
                            assertThat(jBucket.getDocCount(), equalTo(10L));
                            SignificantStringTerms kAgg = jBucket.getAggregations().get("k");
                            assertThat(kAgg.getSubsetSize(), equalTo(10L));
                            for (int k = 0; k < 10; k++) {
                                SignificantStringTerms.Bucket kBucket = kAgg.getBucketByKey(Integer.toString(k));
                                assertThat(jAgg.getSubsetSize(), equalTo(100L));
                                assertThat(kBucket.getDocCount(), equalTo(1L));
                            }
                        }
                    }
                }
            }
        }
    }
    public void testThreeLayerLong() throws IOException {
        try (Directory dir = newDirectory()) {
            try (RandomIndexWriter writer = new RandomIndexWriter(random(), dir)) {
                for (int i = 0; i < 10; i++) {