Support necessary headers in the web server response

[shuding]

This PR adds support of Content-Length, Etag and X-Edge-Runtime headers to the web server.

Bug

• Related issues linked using fixes #number
• Integration tests added
• Errors have helpful link attached, see contributing.md

Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• Related issues linked using fixes #number
• Integration tests added
• Documentation added
• Telemetry added. In case of a feature if it's used or not.
• Errors have helpful link attached, see contributing.md

Documentation / Examples

• Make sure the linting passes by running yarn lint

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	shuding/next.js shu/9efe	Change
buildDuration	15.3s	15.1s	-184ms
buildDurationCached	6.1s	6s	-132ms
nodeModulesSize	484 MB	484 MB	⚠️ +2.6 kB

Page Load Tests Overall increase ✓

	vercel/next.js canary	shuding/next.js shu/9efe	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.046	3.022	-0.02
/ avg req/sec	820.67	827.21	+6.54
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.205	1.172	-0.03
/error-in-render avg req/sec	2075.39	2133.66	+58.27

Client Bundles (main, webpack)

	vercel/next.js canary	shuding/next.js shu/9efe	Change
925.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42 kB	42 kB	✓
main-HASH.js gzip	28.3 kB	28.3 kB	✓
webpack-HASH.js gzip	1.44 kB	1.44 kB	✓
Overall change	72 kB	72 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	shuding/next.js shu/9efe	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	shuding/next.js shu/9efe	Change
_app-HASH.js gzip	1.36 kB	1.36 kB	✓
_error-HASH.js gzip	192 B	192 B	✓
amp-HASH.js gzip	309 B	309 B	✓
css-HASH.js gzip	327 B	327 B	✓
dynamic-HASH.js gzip	3.05 kB	3.05 kB	✓
head-HASH.js gzip	351 B	351 B	✓
hooks-HASH.js gzip	920 B	920 B	✓
image-HASH.js gzip	5.73 kB	5.73 kB	✓
index-HASH.js gzip	263 B	263 B	✓
link-HASH.js gzip	2.36 kB	2.36 kB	✓
routerDirect..HASH.js gzip	320 B	320 B	✓
script-HASH.js gzip	392 B	392 B	✓
withRouter-HASH.js gzip	319 B	319 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	16 kB	16 kB	✓

Client Build Manifests

	vercel/next.js canary	shuding/next.js shu/9efe	Change
_buildManifest.js gzip	460 B	460 B	✓
Overall change	460 B	460 B	✓

Rendered Page Sizes

	vercel/next.js canary	shuding/next.js shu/9efe	Change
index.html gzip	532 B	532 B	✓
link.html gzip	546 B	546 B	✓
withRouter.html gzip	527 B	527 B	✓
Overall change	1.6 kB	1.6 kB	✓

---

Default Build with SWC (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	shuding/next.js shu/9efe	Change
buildDuration	18.1s	18.2s	⚠️ +57ms
buildDurationCached	6s	6s	⚠️ +45ms
nodeModulesSize	484 MB	484 MB	⚠️ +2.6 kB

Page Load Tests Overall increase ✓

	vercel/next.js canary	shuding/next.js shu/9efe	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.04	3.044	0
/ avg req/sec	822.24	821.35	⚠️ -0.89
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.178	1.164	-0.01
/error-in-render avg req/sec	2122	2147.83	+25.83

Client Bundles (main, webpack)

	vercel/next.js canary	shuding/next.js shu/9efe	Change
925.HASH.js gzip	178 B	178 B	✓
framework-HASH.js gzip	42.3 kB	42.3 kB	✓
main-HASH.js gzip	28.7 kB	28.7 kB	✓
webpack-HASH.js gzip	1.45 kB	1.45 kB	✓
Overall change	72.6 kB	72.6 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	shuding/next.js shu/9efe	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	shuding/next.js shu/9efe	Change
_app-HASH.js gzip	1.35 kB	1.35 kB	✓
_error-HASH.js gzip	179 B	179 B	✓
amp-HASH.js gzip	313 B	313 B	✓
css-HASH.js gzip	325 B	325 B	✓
dynamic-HASH.js gzip	3.03 kB	3.03 kB	✓
head-HASH.js gzip	351 B	351 B	✓
hooks-HASH.js gzip	921 B	921 B	✓
image-HASH.js gzip	5.77 kB	5.77 kB	✓
index-HASH.js gzip	261 B	261 B	✓
link-HASH.js gzip	2.44 kB	2.44 kB	✓
routerDirect..HASH.js gzip	322 B	322 B	✓
script-HASH.js gzip	393 B	393 B	✓
withRouter-HASH.js gzip	317 B	317 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	16.1 kB	16.1 kB	✓

Client Build Manifests

	vercel/next.js canary	shuding/next.js shu/9efe	Change
_buildManifest.js gzip	458 B	458 B	✓
Overall change	458 B	458 B	✓

Rendered Page Sizes

	vercel/next.js canary	shuding/next.js shu/9efe	Change
index.html gzip	532 B	532 B	✓
link.html gzip	547 B	547 B	✓
withRouter.html gzip	527 B	527 B	✓
Overall change	1.61 kB	1.61 kB	✓

Commit: 6526909