feat(core): Lookup client on current scope, not hub

[mydea]

One more step to avoid the hub...

Note: This showed a potential footgun (?), where if you pass a client-less scope to startSpan(), no span will be created (because no client = no tracing, and the given scope will be made the current scope in the callback). this is correct behavior here, but could be a bit unexpected ("why is no span being created here??"). We could think about warning if a client-less scope is passed in there, but not sure if that's worth it...

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77.95 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	69.18 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	73.11 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	62.8 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	33.38 KB (+0.02% 🔺)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	33.24 KB (+0.01% 🔺)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.16 KB (+0.01% 🔺)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31.16 KB (+0.03% 🔺)
@sentry/browser - Webpack (gzipped)	22.44 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	76.1 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	67.6 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	33.59 KB (+0.01% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	24.64 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	212.93 KB (0%)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	101.55 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	73.83 KB (0%)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	36.83 KB (+0.01% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	69.55 KB (+0.01% 🔺)
@sentry/react - Webpack (gzipped)	22.46 KB (+0.02% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	87.31 KB (+0.01% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	50.44 KB (+0.01% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	17.11 KB (+0.06% 🔺)

[Lms24]

where if you pass a client-less scope to startSpan()

I still find it weird that we support passing a scope in general but I assume there's a good reason for that (otel?)...

[mydea]

where if you pass a client-less scope to startSpan()

I still find it weird that we support passing a scope in general but I assume there's a good reason for that (otel?)...

the reason for this is to allow to start a span as a child of a specific other span - you can pass a scope that has a current span, which will have the new span attached to it!