TYPO3 Core

Guzzle has the possibility to push custom middleware handler during client initialization.
The T3 core RequestFactory has the protected method "getClient()" and transfers the TYPO3_CONF_VARS HTTP configuration to the Guzzle client.

Guzzle Middleware handler: https://guzzle.readthedocs.io/en/latest/handlers-and-middleware.html#middleware

Current state:
If you define the handler in $GLOBALS['TYPO3_CONF_VARS']['HTTP']['handler'], the default handler stack of guzzle gets overwritten. You are not able to create the default HandlerStack before the Guzzle Client get's initialized.

Should be:
If the configuration $GLOBALS['TYPO3_CONF_VARS']['HTTP']['handler'] is set, the request factory creates the default Guzzle handler stack and pushs the additional handler to the guzzle client.

Code example for the T3 Core RequestFactory:

/**
     * Creates the client to do requests
     * @return ClientInterface
     */
    protected function getClient(): ClientInterface
    {
        $httpOptions = $GLOBALS['TYPO3_CONF_VARS']['HTTP'];
        $httpOptions['verify'] = filter_var($httpOptions['verify'], FILTER_VALIDATE_BOOLEAN, FILTER_NULL_ON_FAILURE) ?? $httpOptions['verify'];

        if (isset($GLOBALS['TYPO3_CONF_VARS']['HTTP']['handler']) && is_array($GLOBALS['TYPO3_CONF_VARS']['HTTP']['handler'])) {
            $stack = \GuzzleHttp\HandlerStack::create();
            foreach ($GLOBALS['TYPO3_CONF_VARS']['HTTP']['handler'] ?? [] as $handler) {
                $stack->push($handler);
            }
            $httpOptions['handler'] = $stack;
        }

        return GeneralUtility::makeInstance(Client::class, $httpOptions);
    }

EDIT: Demo Extension is available on GitHub: https://github.com/JBlocks/t3guzzlelog