Add web vitals monitoring and integrate with GA4

[acrollet]

Are you removing, renaming or moving a folder in this PR?

• No, I'm not changing any folders (skip to TeamSites and delete the rest of this section)
• Yes, I'm removing, renaming or moving a folder

⚠️ TeamSites ⚠️

Examples of a TeamSite: https://va.gov/health and https://benefits.va.gov/benefits/. This scenario is also referred to as the "injected" header and footer. You can reach out in the #sitewide-public-websites Slack channel for questions.

Did you change site-wide styles, platform utilities or other infrastructure?

• No
• Yes, and I used the proxy-rewrite steps to test the injected header scenario

Summary

This PR adds performance reporting in service of 2025 OKR 1.2:

We have reduced the total time Veterans spend waiting for a response from our digital experiences by 50%, towards a goal of less than 4 seconds per transaction.

It uses the web-vitals library to report the core web vitals + TTFB to GA4. This will allow us to view these metrics per app, as our end users experience them. The library is extremely light-weight and should not impact performance itself, as it uses the same mechanism we already use to record analytics events.

The key metric will be LCP, used as a proxy for perceived page load time. TTFB should additionally be useful to help inform how load performance varies with geography.

Testing done

Event collection was verified using the Adswerve extension on a review instance:

Because analytics are not reported from review instances, the next step will be to verify reporting and data shape in the staging environment.

What areas of the site does it impact?

All apps.

Acceptance criteria

Quality Assurance & Testing

• I fixed|updated|added unit tests and integration tests for each feature (if applicable).
• No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs
• Linting warnings have been addressed

Error Handling

• Browser console contains no warnings or errors.
• Events are being sent to the appropriate logging solution
• Feature/bug has a monitor built into Datadog or Grafana (if applicable)

Authentication

• Did you login to a local build and verify all authenticated routes work as expected with a test user

[va-vfs-bot]

ESLint is disabled

vets-website uses ESLint to help enforce code quality. In most situations we would like ESLint to remain enabled.

What you can do

See if the code can be refactored to avoid disabling ESLint, or wait for a VSP review.