ref: Migrate transaction source from metadata to attributes

[lforst]

Moves all of our logic and usage from metadata.source to attributes.'sentry.source'. This is necessary for our migration from transactions to spans.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77.91 KB (-0.04% 🔽)
@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)	62.76 KB (-0.03% 🔽)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	33.37 KB (-0.02% 🔽)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	33.28 KB (-0.01% 🔽)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.24 KB (+0.09% 🔺)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31.25 KB (+0.1% 🔺)
@sentry/browser - Webpack (gzipped)	22.5 KB (+0.14% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	76.26 KB (-0.02% 🔽)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	67.78 KB (-0.04% 🔽)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	33.72 KB (-0.09% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped)	24.81 KB (+0.05% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	213.23 KB (-0.05% 🔽)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	101.85 KB (-0.11% 🔽)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	74.35 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	36.94 KB (+0.05% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	69.51 KB (-0.04% 🔽)
@sentry/react - Webpack (gzipped)	22.54 KB (+0.16% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	87 KB (-0.06% 🔽)
@sentry/nextjs Client - Webpack (gzipped)	50.21 KB (-0.03% 🔽)
@sentry-internal/feedback - Webpack (gzipped)	17.22 KB (+0.12% 🔺)