feat(tracing): Ensure pageload transaction starts at timeOrigin

[mydea]

This adjusts the pageload transaction to ensure its start is always the timeOrigin.
This should help with e.g. lazy loading sentry, where we still want this to be accurate.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	20.61 KB (-0.01% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	64.39 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	19.16 KB (-0.01% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	56.77 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	21.62 KB (0%)
@sentry/browser - Webpack (minified)	72.03 KB (0%)
@sentry/react - Webpack (gzipped + minified)	21.64 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	51.99 KB (+0.02% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	28.15 KB (+0.02% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	26.35 KB (+0.03% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	44.47 KB (-0.01% 🔽)
@sentry/replay - Webpack (gzipped + minified)	38.54 KB (0%)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	63.13 KB (+0.01% 🔺)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	56.22 KB (0%)

[langovoi]

@mydea I think this PR broke some of custom metrics in our project.

We use span.endTimestamp - transaction.startTimestamp to make some custom metrics. After upgrade to 7.46.0 we faced with negative values because span.endTimestamp is milliseconds, but transaction.startTimestamp became microseconds.

Is it expected behavior?

[mydea]

Ugh, sorry about that! I will open a PR to fix this!

[mydea]

Hi @langovoi ,

we've just released 7.49.0 which should fix this issue! sorry for the inconvenience!