Description:

optimizeAt() on DocumentSourceInternalDocumentResultsAndMetadata already elides _metadata when no downstream stage references $$SEARCH_META. The next step is to propagate that signal into the source extension so it can skip issuing the metadata query entirely. Once skip_stream is added to MongoExtensionLogicalAggStageVTable, invoke it from optimizeAt():

  if (!referencesSearchMeta) {
      _metadata = boost::none;
      if (auto* ext = dynamic_cast<DocumentSourceExtensionOptimizable*>(_sourceStage.get())) {
       _sourceStage->skipStream(kMetadataResult);
}
  }