[8.x](backport #40885) libbeat: improve BenchmarkExecHttpRequest with multiple sizes

[mergify]

Proposed commit message

This benchmark uses a hardcoded Hello World response. In a real world scenario responses will range in size, specifically for _bulk responses. Updated the benchmark to consider different sizes when benchmarking this function.

go test -benchmem -run=^$ -tags integration,unit -bench ^BenchmarkExecHTTPRequest$ github.com/elastic/beats/v7/libbeat/esleg/eslegclient -v -count=1

goos: linux
goarch: amd64
pkg: github.com/elastic/beats/v7/libbeat/esleg/eslegclient
cpu: Intel(R) Xeon(R) CPU E5-2697A v4 @ 2.60GHz
BenchmarkExecHTTPRequest
BenchmarkExecHTTPRequest/size:_100
BenchmarkExecHTTPRequest/size:_100/unknown_length
BenchmarkExecHTTPRequest/size:_100/unknown_length-32              468573              2295 ns/op             976 B/op         10 allocs/op
BenchmarkExecHTTPRequest/size:_100/known_length
BenchmarkExecHTTPRequest/size:_100/known_length-32                479560              2313 ns/op             976 B/op         10 allocs/op
BenchmarkExecHTTPRequest/size:_10240
BenchmarkExecHTTPRequest/size:_10240/unknown_length
BenchmarkExecHTTPRequest/size:_10240/unknown_length-32            407080              2770 ns/op             976 B/op         10 allocs/op
BenchmarkExecHTTPRequest/size:_10240/known_length
BenchmarkExecHTTPRequest/size:_10240/known_length-32              400564              2773 ns/op             976 B/op         10 allocs/op
BenchmarkExecHTTPRequest/size:_102400
BenchmarkExecHTTPRequest/size:_102400/unknown_length
BenchmarkExecHTTPRequest/size:_102400/unknown_length-32            97029             10625 ns/op             977 B/op         10 allocs/op
BenchmarkExecHTTPRequest/size:_102400/known_length
BenchmarkExecHTTPRequest/size:_102400/known_length-32             110724             10744 ns/op             976 B/op         10 allocs/op
BenchmarkExecHTTPRequest/size:_1048576
BenchmarkExecHTTPRequest/size:_1048576/unknown_length
BenchmarkExecHTTPRequest/size:_1048576/unknown_length-32            4808            241032 ns/op            1194 B/op         10 allocs/op
BenchmarkExecHTTPRequest/size:_1048576/known_length
BenchmarkExecHTTPRequest/size:_1048576/known_length-32              4701            239657 ns/op            1199 B/op         10 allocs/op

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Relates to Replace the ioutil.ReadAll usage #36151
• Relates to libbeat: use optimized version of io.ReadAll to read _bulk responses #40844

  ---

  This is an automatic backport of pull request libbeat: improve BenchmarkExecHttpRequest with multiple sizes #40885 done by Mergify.

[botelastic]

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