Sentry Performance charges 29 dollars a month plus per-transaction fees for Real User Monitoring. This generates a self-contained JS snippet that collects Core Web Vitals to your sites localStorage - no server, no endpoint, no data collection by anyone else. Paired with a built-in dashboard to view p75 metrics.
Read the story behind this tool: Why I built this - RUM without a data endpoint.
Related: CrUX Field Data Probe / Rendered DOM Paste Audit (paired)