Query Frontend: Update Server-Timing format

[tinitiuset]

What this PR does

This PR updates Server-Timing format to better comply with the standard described here.

The standard does not talk about values, only about duration. But I feel like it is a good idea to follow the same pattern if we return values in this header.

Which issue(s) this PR fixes or relates to

There's no specific issue, but bytes_processed was introduced recently by @madhu-reddy-peram

Checklist

• [x] Tests updated.
• [ ] Documentation added.
• [x] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• [ ] about-versioning.md updated with experimental features.

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