feat(browser): Add option to sample linked traces consistently

[Lms24]

This PR implements consistent sampling across traces as outlined in #15754. For a detailed write up see the issue.

Concrete changes:

• Add sampleLinkedTracesConsistently option (naming TBD but this is what I'm going with for now)
• Emit beforeSampling hook before sampling and creating SentrySpan's in the coreSDK (this hook is already called in Otel-based spans).
• Allow beforeSampling hook subscribers to alter span attributes and parentSampled flag
• Subscribe to beforeSampling hook when sampleLinkedTracesConsistently is enabled and linkPreviousTrace is not disabled
  • modify attributes and parentSampled flag based on the sampling decision, sample rate and sample rand of the previous trace
  • this will ensure that the newly started trace is sampled postitively/negatively depending on the decision of the previous trace
  • we also directly modify the propagation context to ensure that the previous trace's sampling decision, rate and rand are propagated correctly to downstream services and in the trace envelope header. This is necessary to ensure correct span metrics extrapolation.

[Copilot]

Copilot reviewed 20 out of 20 changed files in this pull request and generated 1 comment.

[github-actions]

size-limit report 📦

Path	Size	% Change	Change
@sentry/browser	23.3 KB	+0.08%	+18 B 🔺
@sentry/browser - with treeshaking flags	23.14 KB	+0.09%	+20 B 🔺
⛔️ @sentry/browser (incl. Tracing) (max: 37.11 KB)	37.15 KB	+0.55%	+206 B 🔺
⛔️ @sentry/browser (incl. Tracing, Replay) (max: 74.22 KB)	74.41 KB	+0.37%	+275 B 🔺
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	67.76 KB	+0.38%	+260 B 🔺
@sentry/browser (incl. Tracing, Replay with Canvas)	79.06 KB	+0.33%	+260 B 🔺
@sentry/browser (incl. Tracing, Replay, Feedback)	90.9 KB	+0.32%	+295 B 🔺
@sentry/browser (incl. Feedback)	39.68 KB	+0.02%	+7 B 🔺
@sentry/browser (incl. sendFeedback)	27.92 KB	+0.09%	+24 B 🔺
@sentry/browser (incl. FeedbackAsync)	32.69 KB	+0.06%	+18 B 🔺
@sentry/react	25.11 KB	+0.08%	+19 B 🔺
@sentry/react (incl. Tracing)	39.08 KB	+0.56%	+220 B 🔺
@sentry/vue	27.57 KB	+0.23%	+63 B 🔺
@sentry/vue (incl. Tracing)	38.89 KB	+0.59%	+231 B 🔺
@sentry/svelte	23.33 KB	+0.06%	+12 B 🔺
CDN Bundle	24.53 KB	+0.08%	+18 B 🔺
CDN Bundle (incl. Tracing)	37.19 KB	+0.66%	+246 B 🔺
CDN Bundle (incl. Tracing, Replay)	72.25 KB	+0.36%	+263 B 🔺
CDN Bundle (incl. Tracing, Replay, Feedback)	77.4 KB	+0.33%	+253 B 🔺
CDN Bundle - uncompressed	71.52 KB	+0.07%	+46 B 🔺
CDN Bundle (incl. Tracing) - uncompressed	109.93 KB	+0.65%	+722 B 🔺
CDN Bundle (incl. Tracing, Replay) - uncompressed	221.22 KB	+0.32%	+722 B 🔺
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	233.76 KB	+0.31%	+722 B 🔺
@sentry/nextjs (client)	40.72 KB	+0.57%	+236 B 🔺
⛔️ @sentry/sveltekit (client) (max: 37.6 KB)	37.62 KB	+0.62%	+235 B 🔺
@sentry/node	143.25 KB	+0.02%	+22 B 🔺
@sentry/node - without tracing	96.5 KB	+0.03%	+27 B 🔺
@sentry/aws-serverless	120.79 KB	+0.02%	+20 B 🔺

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