ref(core): Use sdkProcessingMetadata field for transaction sampling data

[lobsterkatie]

In #4252, a new sdkProcessingMetadata field was added to the Scope and Event interfaces, to provide a place to put data which is needed for in-SDK event processing but shouldn't be sent to Sentry. This refactors one step in that processing... process... to use the new field, eliminating the need to pull the data out of debug_meta before the event is sent.

[github-actions]

size-limit report

Path	Base Size (c1341fd)	Current Size	Change
@sentry/browser - CDN Bundle (gzipped)	19.75 KB	19.63 KB	-0.57% 🔽
@sentry/browser - CDN Bundle (minified)	62.86 KB	62.51 KB	-0.56% 🔽
@sentry/browser - Webpack	22.29 KB	22.16 KB	-0.57% 🔽
@sentry/browser - Webpack - gzip = false	76.11 KB	75.66 KB	-0.6% 🔽
@sentry/react - Webpack	22.32 KB	22.2 KB	-0.57% 🔽
@sentry/nextjs Client - Webpack	46.46 KB	46.41 KB	-0.11% 🔽
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	28.41 KB	28.36 KB	-0.19% 🔽