Vercel Metric Drain Support (Speed Insights, Web Analytics) for Sentry Metrics

You can now send trace-connected metrics to Sentry! See the docs [here](https://docs.sentry.io/product/explore/metrics/).

This issue tracks support metric drains from Vercel, specifically for [Speed Insights](https://vercel.com/docs/drains/reference/speed-insights) and [Web Analytics](https://vercel.com/docs/drains/reference/analytics). If you're interested in Vercel Metric Drain support, please react to this issue and leave a comment about your use case. That will help us prioritize this accordingly.

Looking for Vercel Log/Trace Drains? See the docs [here](https://docs.sentry.io/product/drains/integration/vercel/
).

Start by reading the linked Vercel Speed Insights and Web Analytics drain references, then compare them with Sentry's existing Vercel Log/Trace Drains documentation. Clarify the supported metric payloads, integration entry points, tests, and acceptance criteria before implementation; done means documented and working support for the requested Vercel metric drains.

Written by the indexing model from the issue text.