Support custom headers in Traefik Logs, e.g. Cloudflare's cf-connecting-ip

[nktnet1]

What problem will this feature address?

When using cloudflare domains with proxy turned on, the client's IP is passed in the header with the key CF-Connecting-IP

• https://developers.cloudflare.com/fundamentals/reference/http-headers/#cf-connecting-ip

We have been receiving many 404s to paths that do not exist in our applications.
Currently, the Traefik request logs only shows the IP of the client, which would be Cloudflare.

Describe the solution you'd like

It would be helpful if we can also extract custom values from the header to include in the logs.
That way, we can include the CF-Connecting-IP.

Describe alternatives you've considered

Printing out the IP at the application level

Additional context

No response

Will you send a PR to implement it?

No