Standalone http.client spans

[cleptric]

Many users have been confused and frustrated by the fact that after the automatically created page load transaction is finished, XMLHttpRequest and fetch requests are only automatically instrumented with the user explicitly wrapping these requests in a transaction.

For v8, we want to change this by utilizing single-span ingestion. By default, each request will be traced and sent to Sentry as a segment span (span.isSegment: true,). The user can turn off the behaviour with a new Sentry.browserTracingIntegration() option.

Tasks

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1. Create/send span envelope (already done via feat(tracing): Adds span envelope and datacategory #11534)
2. feat(core): Add options to start standalone (segment) spans via start*Span APIs #11696
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3. feat(core): Add trace envelope header to span envelope #11699
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4. Adjust fetch instrumentation (feat(browser): Send standalone fetch and XHR spans if there's no active parent span #11783)
5. Adjust xhr instrumentation (feat(browser): Send standalone fetch and XHR spans if there's no active parent span #11783)

[Lms24]

http.client spans count as segment spans and they need to have the following properties:

segmentId = spanId
isSegment: true

In contrast, INP spans don't need to set segment properties because we don't care about them not showing up in the trace view.

[Lms24]

Closing this at the moment as this is officially on hold until further notice. The experimental standalone option still exists but XHR and fetch request spans are still only created if there's a parent segment/root span. The PRs to send standalone fetch and XHR spans were reverted.