Fix perf spans

[javivelasco]

• Remove a not-needed performance.measure() call.
• Fixes reporting client component loading span to happen on headers flush.
• Simplifies measures by using performance.now()

Closes NEXT-2546

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	javivelasco/next.js fix-perf-spans	Change
buildDuration	14.4s	14.5s	N/A
buildDurationCached	8s	7.8s	N/A
nodeModulesSize	197 MB	197 MB	⚠️ +58.7 kB
nextStartRea..uration (ms)	404ms	409ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	javivelasco/next.js fix-perf-spans	Change
1068-HASH.js gzip	30.3 kB	30.3 kB	N/A
3f784ff6-HASH.js gzip	53.5 kB	53.5 kB	N/A
4944-HASH.js gzip	5.04 kB	5.03 kB	N/A
8423.HASH.js gzip	181 B	181 B	✓
framework-HASH.js gzip	45.2 kB	45.2 kB	✓
main-app-HASH.js gzip	241 B	243 B	N/A
main-HASH.js gzip	32.2 kB	32.1 kB	N/A
webpack-HASH.js gzip	1.7 kB	1.7 kB	✓
Overall change	47.1 kB	47.1 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	javivelasco/next.js fix-perf-spans	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	javivelasco/next.js fix-perf-spans	Change
_app-HASH.js gzip	196 B	196 B	✓
_error-HASH.js gzip	184 B	183 B	N/A
amp-HASH.js gzip	503 B	504 B	N/A
css-HASH.js gzip	323 B	324 B	N/A
dynamic-HASH.js gzip	2.5 kB	2.51 kB	N/A
edge-ssr-HASH.js gzip	258 B	259 B	N/A
head-HASH.js gzip	353 B	351 B	N/A
hooks-HASH.js gzip	370 B	370 B	✓
image-HASH.js gzip	4.21 kB	4.2 kB	N/A
index-HASH.js gzip	259 B	259 B	✓
link-HASH.js gzip	2.68 kB	2.67 kB	N/A
routerDirect..HASH.js gzip	313 B	314 B	N/A
script-HASH.js gzip	386 B	385 B	N/A
withRouter-HASH.js gzip	309 B	311 B	N/A
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	931 B	931 B	✓

Client Build Manifests

	vercel/next.js canary	javivelasco/next.js fix-perf-spans	Change
_buildManifest.js gzip	485 B	484 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	javivelasco/next.js fix-perf-spans	Change
index.html gzip	529 B	527 B	N/A
link.html gzip	541 B	539 B	N/A
withRouter.html gzip	524 B	522 B	N/A
Overall change	0 B	0 B	✓

Edge SSR bundle Size

	vercel/next.js canary	javivelasco/next.js fix-perf-spans	Change
edge-ssr.js gzip	2.29 kB	2.29 kB	N/A
page.js gzip	2.98 kB	2.98 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	javivelasco/next.js fix-perf-spans	Change
middleware-b..fest.js gzip	623 B	626 B	N/A
middleware-r..fest.js gzip	151 B	151 B	✓
middleware.js gzip	466 B	465 B	N/A
edge-runtime..pack.js gzip	839 B	839 B	✓
Overall change	990 B	990 B	✓

Next Runtimes Overall increase ⚠️

	vercel/next.js canary	javivelasco/next.js fix-perf-spans	Change
app-page-exp...dev.js gzip	167 kB	167 kB	N/A
app-page-exp..prod.js gzip	96.1 kB	96.2 kB	N/A
app-page-tur..prod.js gzip	97.9 kB	97.9 kB	N/A
app-page-tur..prod.js gzip	92.3 kB	92.4 kB	N/A
app-page.run...dev.js gzip	136 kB	136 kB	N/A
app-page.run..prod.js gzip	90.8 kB	90.9 kB	N/A
app-route-ex...dev.js gzip	22.1 kB	22.1 kB	✓
app-route-ex..prod.js gzip	14.9 kB	14.9 kB	✓
app-route-tu..prod.js gzip	14.9 kB	14.9 kB	✓
app-route-tu..prod.js gzip	14.7 kB	14.7 kB	✓
app-route.ru...dev.js gzip	21.8 kB	21.8 kB	✓
app-route.ru..prod.js gzip	14.7 kB	14.7 kB	✓
pages-api-tu..prod.js gzip	9.49 kB	9.49 kB	✓
pages-api.ru...dev.js gzip	9.76 kB	9.76 kB	✓
pages-api.ru..prod.js gzip	9.48 kB	9.48 kB	✓
pages-turbo...prod.js gzip	22.2 kB	22.3 kB	⚠️ +158 B
pages.runtim...dev.js gzip	22.8 kB	23 kB	⚠️ +159 B
pages.runtim..prod.js gzip	22.2 kB	22.3 kB	⚠️ +157 B
server.runti..prod.js gzip	50.2 kB	50.3 kB	⚠️ +159 B
Overall change	249 kB	250 kB	⚠️ +633 B

build cache

	vercel/next.js canary	javivelasco/next.js fix-perf-spans	Change
0.pack gzip	1.49 MB	1.49 MB	N/A
index.pack gzip	105 kB	104 kB	N/A
Overall change	0 B	0 B	✓

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Commit: 792a5b1

[XoL1507]

thank for everyone's support