test(nextjs): Fix flaky nextjs-16 metrics tests - #25517

The Should emit metrics from server and client test in nextjs-16 intermittently failed because the client test.page.count metric had no span_id. Metrics only get a span_id when a span is active at capture time (packages/core/src/metrics/internal.ts), and the click handler relied on the pageload idle span still being open when Playwright clicked. That's a timing race, and the slower next dev --webpack run is where it keeps losing: all four recent failures came from the development-webpack pass.

The metrics page now emits the client metrics inside an explicit Sentry.startSpan, so a span is always active and the existing span_id assertions stay strict. The same fix is applied to nextjs-16-static, which has an identical page and assertions.

via Francesco Gringl-Novy.

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