Add missing bytes field to backend response log

[malud]

Add bytes log field to response field for backend logs.
This is done by collecting and delaying the backend log until the beresp body is read from the endpoint handler (proxy / copybody) or eval reads.
Fired right after the access-log.

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Reviewer checklist

• Read PR description: a summary about the changes is required
• Changelog updated
• Documentation: docs/{Reference, Cli, ...}, Docker and cli help/usage
• Pulled branch, manually tested
• Verified requirements are met
• Reviewed the code
• Reviewed the related tests