Add traffic size metrics

[tomMoulard]

What does this PR do?

This PR superseeds the PR #8432 from @OmarElawady by adding requests_bytes and responses_bytes metrics for all metric backend.

It refactors the access log middleware to use the new capture middleware.

This new capture middleware extracts request/response size and stores it in the request context.
There might be a memory leak introduced by using the http.Request contexts: golang/go#50798

As pilot is deprecated, this PR does not add support for this metric backend.

Motivation

This PR resolves #7804.

More

• Added/updated tests
• Added/updated documentation

Additional Notes

Benchmarks with and without metrics

The following output display benchmarks of the capture middleware were:

• captured measure the response size
• body measure the response and request size

$ go test -bench=. ./pkg/middlewares/capture/
goos: linux
goarch: amd64
pkg: github.com/traefik/traefik/v2/pkg/middlewares/capture
cpu: Intel(R) Core(TM) i7-10750H CPU @ 2.60GHz
BenchmarkCapture/2k-12				280507	 4015 ns/op	 510.03 MB/s 	 5072 B/op	14 allocs/op
BenchmarkCapture/20k-12				135726	 8301 ns/op	2467.26 MB/s 	41936 B/op	14 allocs/op
BenchmarkCapture/100k-12			 45494	26059 ns/op	3929.54 MB/s	213968 B/op	14 allocs/op
BenchmarkCapture/2k_captured-12			263713	 4356 ns/op	 470.20 MB/s	  5552 B/op	18 allocs/op
BenchmarkCapture/20k_captured-12		132243	 8790 ns/op	2329.98 MB/s	 42416 B/op	18 allocs/op
BenchmarkCapture/100k_captured-12	 	45650	26587 ns/op	3851.57 MB/s	214448 B/op	18 allocs/op
BenchmarkCapture/2k_body-12			274135	 7471 ns/op	 274.12 MB/s 	 5624 B/op	20 allocs/op
BenchmarkCapture/20k_body-12			130206	21149 ns/op	 968.36 MB/s	 42488 B/op	20 allocs/op
BenchmarkCapture/100k_body-12			 41600	51716 ns/op	1980.06 MB/s	214520 B/op	20 allocs/op

Co-authored-by: OmarElawady omarelawady1998@gmail.com
Co-authored-by: Mathieu Lonjaret mathieu.lonjaret@gmail.com
Co-authored-by: Romain rtribotte@users.noreply.github.com

[rtribotte]

LGTM 👌

[kevinpollet]

LGTM 👍