feat(spans): Extract INP metrics from spans

[phacops]

Fixes #2873.

[jjbayer]

Do we know what volume we're expecting for this change initially? Do SDKs already send this measurement in practice?

See comment about strings.py.

[phacops]

SDKs are not sending this measurement yet, volume will be 0 until the next release.

[jjbayer]

This turned out hackier than I thought, with all the cloning. We could instead create the pseudo-event before the retain_items loop, pass that into normalize() instead, and .take() measurements & spans in and out of that to prevent cloning. But I would not invest time into that, we should instead have a generic normalize_performance_score function that works on both spans and events.