fix(core): Ensure standalone spans respect sampled flag

[AbhiPrasad]

@edwardgou-sentry pointed out that sendSpanEnvelope immediately sends spans without checking for this._sampled, which means you can't use tracesSampler to filter them. This changes that!

We also record client outcomes for sample_rate with spans.

[edwardgou-sentry]

Thanks!

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser	22.03 KB (0%)
@sentry/browser (incl. Tracing)	33.23 KB (+0.08% 🔺)
@sentry/browser (incl. Tracing, Replay)	69 KB (+0.08% 🔺)
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	62.31 KB (+0.07% 🔺)
@sentry/browser (incl. Tracing, Replay with Canvas)	73.07 KB (+0.09% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback)	85.23 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback, metrics)	87.05 KB (+0.04% 🔺)
@sentry/browser (incl. metrics)	26.24 KB (+0.11% 🔺)
@sentry/browser (incl. Feedback)	38.24 KB (0%)
@sentry/browser (incl. sendFeedback)	26.65 KB (0%)
@sentry/browser (incl. FeedbackAsync)	31.22 KB (0%)
@sentry/react	24.77 KB (0%)
@sentry/react (incl. Tracing)	36.28 KB (+0.09% 🔺)
@sentry/vue	26.06 KB (+0.1% 🔺)
@sentry/vue (incl. Tracing)	35.09 KB (+0.08% 🔺)
@sentry/svelte	22.17 KB (0%)
CDN Bundle	23.36 KB (0%)
CDN Bundle (incl. Tracing)	34.9 KB (+0.03% 🔺)
CDN Bundle (incl. Tracing, Replay)	69.03 KB (+0.06% 🔺)
CDN Bundle (incl. Tracing, Replay, Feedback)	74.21 KB (+0.03% 🔺)
CDN Bundle - uncompressed	68.68 KB (0%)
CDN Bundle (incl. Tracing) - uncompressed	103.34 KB (+0.06% 🔺)
CDN Bundle (incl. Tracing, Replay) - uncompressed	213.81 KB (+0.04% 🔺)
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	226.4 KB (+0.04% 🔺)
@sentry/nextjs (client)	36.17 KB (+0.08% 🔺)
@sentry/sveltekit (client)	33.86 KB (+0.09% 🔺)
@sentry/node	112.67 KB (0%)
@sentry/node - without tracing	90.07 KB (0%)
@sentry/aws-serverless	99.16 KB (0%)