Re: [w3ctag/design-reviews] Event-Level Click Conversion Measurement API (#418)

Hey TAG reviewers,
Sorry for the delay. Here is our conclusions from ongoing discussions with @johnwilander:
- **Naming**: our attribute names are quite different from PCM and we’re working on a new set of names after [receiving some feedback](https://github.com/WICG/conversion-measurement-api/issues/57) that should hopefully make things more clear. We’ll update that issue with the proposed names but it may be something we can align on.
- **Bits**: PCM is [potentially interested](https://github.com/privacycg/private-click-measurement/issues/28) in supplying 3 bits of conversion data (aligning with this proposal) but we are still interested in event-level data on the impression side. We agreed that the 64 bit ID could live in a different attribute in the Privacy CG F2F in May.
- **Noise**: We are still interested in adding noise to our 3 bits but they are not. This could be resolved in a spec by having noise be browser determined and the noise rate explicitly included in the conversion report.
- **Report delays**: There are some fundamental privacy reasons why we can’t implement the same reporting delays as PCM due to our 64 bits, so we can’t quite align there. We could get around this in the spec by having delays be browser controlled, and having the reports say something about which delays were imposed by the UA.
- **3rd party reporters**: we proposed a [possible way](https://github.com/privacycg/private-click-measurement/issues/48#issuecomment-677884248) to align on API surface but PCM is not interested in supporting this use case in general and would only support reports going to publishers / advertisers directly.
- **Multiple conversions per click**: PCM is not interested in supporting multiple conversions per click if they occur >24 hrs after the first click, which makes it difficult to align since this is an important use case we want to support. This feature also influences our proposal for report delays. This is discussed in [this issue](https://github.com/privacycg/private-click-measurement/issues/29#issuecomment-567624666).

cc @johnwilander @michaelkleber @johnivdel. Happy to discuss some of these items more in the next privacy CG F2F if it's useful.

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