feat(core): Add span.isRecording() instead of span.sampled

[mydea]

To align this with OTEL API.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	76.14 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	67.52 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	61.15 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	32.13 KB (+0.08% 🔺)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	30.49 KB (0%)
@sentry/browser - Webpack (gzipped)	22.21 KB (0%)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	73.59 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	65.23 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	31.42 KB (+0.05% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	23.36 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	204.48 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	94.37 KB (+0.08% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	69.23 KB (0%)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	34.39 KB (+0.06% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	67.93 KB (+0.04% 🔺)
@sentry/react - Webpack (gzipped)	22.23 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	84.6 KB (+0.04% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	49.19 KB (+0.05% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	16.73 KB (0%)