spec average-code-fetch-time and participating-ig-count (WICG#1328)

* Reserved.once specification.

* Some metrics stuff, but it's a bit off for re-run, so need to think some
more.

Actually implement the get.

* Fix handwaveyness of fetch metrics.

* Fix the re-run thing, and a bit of prep for later.

* Fix formatting.

* Fix bug + misc found in feedback.

* Typo fix.

* k-anon only. The impl gets it wrong.

* Fix participating IG count computation.

* Back to always.

* Fix the reportAdditionalWin case.

* Improve report-win enum at suggestion.

* Apply feedback

* Apply feedback

* + PA contributions type

* More feedback

* More phrasing tweaks.

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Co-authored-by: Maks Orlovich <[email protected]>