Symfony version(s) affected: 5.3.2

Description
As menstionned here #33311 (comment) guzzle output the request headers (including default headers in configuration).
Unfortunatly even with this PR #35407 we still not collecting all headers : headers defined in configuration are missing!

How to reproduce

#config/packages/framework.yaml
http_client:
        default_options:
            headers:
                Accept: application/json
                User-Agent: "app_web"
        scoped_clients:
            http.client.api:
                base_uri: '%api_uri%'
                headers:
                    CustomHeader: "%api_host%"

Possible Solution
Exposing $defaultHeaders attribute and get it from HttpClientTrait directly as an attribute.

Additional context