**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
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
 */
package org.elasticsearch.search.aggregations.bucket.terms;
import org.apache.lucene.analysis.standard.StandardAnalyzer;
import org.apache.lucene.document.BinaryDocValuesField;
import org.apache.lucene.document.Document;
import org.apache.lucene.document.Field;
import org.apache.lucene.document.SortedDocValuesField;
import org.apache.lucene.document.SortedNumericDocValuesField;
import org.apache.lucene.document.StoredField;
import org.apache.lucene.index.DirectoryReader;
import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.IndexWriter;
import org.apache.lucene.index.IndexWriterConfig;
import org.apache.lucene.index.Term;
import org.apache.lucene.search.IndexSearcher;
import org.apache.lucene.search.MatchAllDocsQuery;
import org.apache.lucene.search.TermQuery;
import org.apache.lucene.store.Directory;
import org.apache.lucene.tests.index.RandomIndexWriter;
import org.apache.lucene.util.BytesRef;