code: public function client(callable $handler = null): ClientInterface
      {
          $handlerStack = HandlerStack::create($handler);
          $handlerStack->push(
              Middleware::log($this->logger, new MessageFormatter($this->template))
          );
          return new Client([
              'handler' => $handlerStack,
          ]);
      }
docs: @param callable|null $handler
  
  @return ClientInterface