code: protected function getAllSearchTargets()
      {
          if ($this->endpointResolver instanceof SingleEndpointResolver && !$this->endpointResolver->hasMultipleEndpoints()) {
              return '';
          }
  
          $shards = [];
          $searchTargets = $this->endpointResolver->getEndpoints();
          if (!empty($searchTargets)) {
              foreach ($searchTargets as $endpointName) {
                  $shards[] = $this->endpointRegistry->getEndpoint($endpointName)->getIdentifier();
              }
          }
  
          return  implode(',', $shards);
      }
docs: Returns all search targets without language constraint.
  
  Only return endpoints if there are more then one configured, as this is meant for use on shard parameter.
  
  @return string