server-timing endpoint sequences

[johakoch]

create a result channel for every item, so that result map contains result from every item, so that server-timing uses all backend request times

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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