Support Element Timing API in Browser SDK

[mydea]

Description

We want to add support for https://developer.mozilla.org/en-US/docs/Web/API/PerformanceElementTiming in the Browser SDK.

[Lms24]

Some initial thoughts:

• The API is really cool and allows greater user input than e.g. LCP and we could probably add it to BrowserTracing
• The element loading time seems to be measured from performance.timeOrigin onwards
• No guarantees that the tagged element is loaded within the pageload span. So it probably makes sense to send this span standalone
• Really just an idea but could we inject the elementtiming attribute into user-html via bundler plugins?

Quick experiment: