browser-integration-tests: port request and web vitals suites to span streaming

Follow the porting recipe in the parent issue.

`http.client` and resource span names are low cardinality under streaming, and web vitals arrive as standalone streamed spans rather than as measurements on a transaction. Expect assertion shape changes, not just helper swaps.

## Suites

* `tracing/request/*` (25 pinned files)
* `tracing/metrics/*` (11 pinned files, web vitals and resource spans)
* `tracing/interactions/*`
* `tracing/user-timing/*`

## Twins to fold back

* `tracing/request`: `fetch-streamed`, `xhr-streamed`, `fetch-relative-url-streamed`, `xhr-relative-url-streamed`, `fetch-streamed-track-stream-performance`
* `tracing/metrics`: `web-vitals-lcp-streamed-spans`, `web-vitals-cls-streamed-spans`, `web-vitals-inp-streamed-spans`, `web-vitals-ttfb-streamed`, `pageload-resource-spans-streamed`