Log X-Gitlab-* header fields

The monolith now sends instance and user IDs as well as gitlab-realm with the HTTP request. This will be used to count unique users and distinguish self-managed from SaaS requests.

Related issues/MRs:

• https://gitlab.com/gitlab-org/gitlab/-/issues/416621
• gitlab-org/gitlab!127882 (merged)
• gitlab-org/gitlab!128998 (merged)

Example:

{
    "url": "http://ai-gateway:5000/v2/completions",
    "path": "/v2/completions",
...
    "gitlab_instance_id": "9ebada7a-f5e2-477a-8609-17797fa95cb9",
    "gitlab_global_user_id": "XTuMnZ6XTWkP3yh0ZwXualmOZvm2Gg/bk9jyfkL7Y6k=",
    "gitlab_realm": "self-managed",
}