**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
SignificantTermsAggregationBuilder.BACKGROUND_FILTER
        );
        PARSER.declareField(
            (b, v) -> b.includeExclude(IncludeExclude.merge(v, b.includeExclude())),
            IncludeExclude::parseInclude,
            IncludeExclude.INCLUDE_FIELD,
            ObjectParser.ValueType.OBJECT_ARRAY_OR_STRING
        );
        PARSER.declareField(
            (b, v) -> b.includeExclude(IncludeExclude.merge(b.includeExclude(), v)),
            IncludeExclude::parseExclude,
            IncludeExclude.EXCLUDE_FIELD,
            ObjectParser.ValueType.STRING_ARRAY
        );
    }
    public static SignificantTermsAggregationBuilder parse(String aggregationName, XContentParser parser) throws IOException {
        return PARSER.parse(parser, new SignificantTermsAggregationBuilder(aggregationName), null);
    }
    public static void registerAggregators(ValuesSourceRegistry.Builder builder) {
        SignificantTermsAggregatorFactory.registerAggregators(builder);
    }
    private IncludeExclude includeExclude = null;
    private String executionHint = null;
    private QueryBuilder filterBuilder = null;
    private TermsAggregator.BucketCountThresholds bucketCountThresholds = new BucketCountThresholds(DEFAULT_BUCKET_COUNT_THRESHOLDS);
    private SignificanceHeuristic significanceHeuristic = DEFAULT_SIGNIFICANCE_HEURISTIC;
    public SignificantTermsAggregationBuilder(String name) {
        super(name);
    }
    /**