Browser events migration from body fields to attributes

[jaredDlewis]

Fixes #3401

Changes

Per #1870, events should not be using body fields.

This PR moves the web vital fields from the event body to browser.web_vital.* attributes.

• Here is a link to the instrumentation for these web vitals: https://github.com/open-telemetry/opentelemetry-browser/blob/main/packages/instrumentation/src/web-vitals/instrumentation.ts#L124

Important

Pull request acceptance is subject to the triage process as described in Issue and PR Triage Management.
PRs that do not follow the guidance above may be automatically rejected and closed.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Links to prototypes or existing instrumentations (when adding or changing conventions)
• Disclose AI usage, see OTel GenAI policy:
  • no AI used
  • AI-assisted
  • bulk AI-generated
• I have the experience and knowledge necessary to understand, review, and validate all content in this PR.¹

Footnotes

1. Yes, I can answer maintainer questions about the content of this PR without using AI. ↩