feat(core): Fix span scope handling & transaction setting

[mydea]

This does the following things:

1. Instead of setting transaction tag, we ensure to set the transaction on the event itself.
2. This also allows us to remove code that lead to other bugs, which is that we needed to fork the scope for startInactiveSpan just to get the correct transaction tag.
3. Add tests to ensure that the scope is correctly applied when using startInactiveSpan

Background

I initially implemented startInactiveSpan to fork the current scope & set the span on that to satisfy two things:

1. We do not want to set this span as active on the actual current scope
2. We needed a way to set the transaction tag on the span (if it was a transaction) in an event processor

For 2., in order for getActiveSpan() to work in there I had to make the span active in the forked scope.

This kind of worked, BUT I now noticed that this has the massive problem:

const scope = getCurrentScope();
const span = startInactiveSpan(...);
scope.setTag(...); // <-- this will not be applied to the span!

So I went in and fixed this - and since we talked about not needing to set the transaction tag anyhow (instead, just set the transaction itself!), I figured I can fix these in one go.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77.38 KB (-0.03% 🔽)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	68.59 KB (-0.03% 🔽)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	72.53 KB (-0.03% 🔽)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	62.15 KB (-0.03% 🔽)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	32.82 KB (-0.06% 🔽)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	32.82 KB (-0.06% 🔽)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.06 KB (-0.01% 🔽)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31.06 KB (0%)
@sentry/browser - Webpack (gzipped)	22.3 KB (-0.01% 🔽)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	75.82 KB (-0.02% 🔽)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	67.45 KB (-0.02% 🔽)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	33.29 KB (+0.01% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	24.79 KB (-0.02% 🔽)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	211.08 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	99.82 KB (+0.01% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	74.08 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	36.34 KB (-0.04% 🔽)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	68.87 KB (-0.03% 🔽)
@sentry/react - Webpack (gzipped)	22.33 KB (-0.01% 🔽)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	85.35 KB (-0.02% 🔽)
@sentry/nextjs Client - Webpack (gzipped)	49.68 KB (-0.03% 🔽)
@sentry-internal/feedback - Webpack (gzipped)	17.09 KB (0%)

[mydea]

@AbhiPrasad I noticed we are also setting transaction as tag on metrics summaries, not sure if we can/should also change this (I left it for now)