refactor(web-vitals): require explicit trackingLevel for all metrics

[overbalance]

What problem is this solving?

Makes the trackingLevel option require explicit 'all' value to track FCP and TTFB, aligning with test expectations.

Short description of changes

• Changed condition from checking trackingLevel === 'core' to trackingLevel === 'all'
• CORE_WEB_VITALS (CLS, INP, LCP) tracked unless explicitly set to 'all'
• ALL_WEB_VITALS (CLS, INP, LCP, FCP, TTFB) requires trackingLevel: 'all'

Testing

• Existing tests verify this behavior and pass

[github-actions]

Chrome DevTools Protocol Tracing (Script: 107.10ms, Heap: 13.58MB)

	Number of Requests	Size of Requests	Script Duration	Task Duration	Heap Used Size
Requests	+3 requests	+29.72 KB
Page Loaded			+14.28 ms	+4.87 ms	+0.95 MB
Generate 100 fetch requests			+16.17 ms	+60.22 ms	+1.76 MB
Generate 100 XHR requests			+41.59 ms	+121.29 ms	+2.35 MB
Click 100 buttons and generate 100 logs			+24.21 ms	+35.65 ms	+2.95 MB
Throw a 100 exceptions			+3.39 ms	+15.31 ms	+2.45 MB
End Session			+7.45 ms	+14.91 ms	+3.13 MB
Total	+3 requests	+29.72 KB	+107.10 ms	+252.25 ms	+13.58 MB

Lighthouse (Script Eval: 46.02ms)

	Difference	Description
Total Blocking Time	0 ms	Difference in Total Blocking Time: Sum of all time periods between FCP and Time to Interactive, when task length exceeded 50ms, expressed in milliseconds. Learn more about the Total Blocking Time metric.
Main Thread Time	+63.65 ms	Difference in Main Thread Time: Consider reducing the time spent parsing, compiling and executing JS. You may find delivering smaller JS payloads helps with this. Learn how to minimize main-thread work
Script Evaluation Time	+46.02 ms	Difference in Script Evaluation Time: Consider reducing the time spent parsing, compiling, and executing JS. You may find delivering smaller JS payloads helps with this. Learn how to reduce Javascript execution time.

Platform Tests (vite-7 gzip: 47.99KB)

vite-7 Platform Tests

	Total Uncompressed Size	Total Gzip Size
vite-7 - esnext	+164.67 KB	+47.99 KB
vite-7 - es2015	+171.51 KB	+49.67 KB

vite-otel-latest Platform Tests

	Total Uncompressed Size	Total Gzip Size
vite-otel-latest - esnext	+163.74 KB	+49.69 KB
vite-otel-latest - es2015	+170.50 KB	+51.32 KB

webpack-5 Platform Tests

	Total Uncompressed Size	Total Gzip Size
webpack-5 - esnext	+121.11 KB	+43.17 KB
webpack-5 - es2015	+122.22 KB	+44.65 KB