**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
@Override
    protected SignificantTermsAggregationBuilder shallowCopy(AggregatorFactories.Builder factoriesBuilder, Map<String, Object> metadata) {
        return new SignificantTermsAggregationBuilder(this, factoriesBuilder, metadata);
    }
    @Override
    protected AggregationBuilder doRewrite(QueryRewriteContext queryRewriteContext) throws IOException {
        if (filterBuilder != null) {
            QueryBuilder rewrittenFilter = filterBuilder.rewrite(queryRewriteContext);
            if (rewrittenFilter != filterBuilder) {
                SignificantTermsAggregationBuilder rewritten = shallowCopy(factoriesBuilder, metadata);
                rewritten.backgroundFilter(rewrittenFilter);
                return rewritten;
            }
        }
        return super.doRewrite(queryRewriteContext);
    }
    @Override
    protected void innerWriteTo(StreamOutput out) throws IOException {
        bucketCountThresholds.writeTo(out);
        out.writeOptionalString(executionHint);
        out.writeOptionalNamedWriteable(filterBuilder);
        out.writeOptionalWriteable(includeExclude);
        out.writeNamedWriteable(significanceHeuristic);
    }
    @Override
    protected boolean serializeTargetValueType(Version version) {
        return true;
    }
    public TermsAggregator.BucketCountThresholds getBucketCountThresholds() {
        return new TermsAggregator.BucketCountThresholds(bucketCountThresholds);