Report LCP and INP subparts on web vital spans #25337

LCP and INP only tell us a page is slow, not why. Both break down into phases that each point at a different fix, existing RUM tools started exposing them recently, so I think we should too.

The subparts apply to LCP and INP:

• LCP: TTFB, resource load delay, resource load duration, element render delay
• INP: input delay, processing duration, presentation delay

We already have the raw timestamps for all of them. The LCP span in browser-utils/src/web-vitals/spans.ts carries load_time and render_time, and the INP span already has the PerformanceEventTiming entry, so it's mostly subtraction.

The attribute names are proposed in getsentry/sentry-conventions#712, so this waits on that merging. They go on the LCP/INP spans now and carry over as attributes on the web vital metrics later. TTFB isn't duplicated on LCP since it's lcp.value minus the other three.

Refs:

• Optimize LCP
• Optimize INP