Add OTLP write handler benchmark

[SungJin1212]

This PR adds a benchmark for the OTLP write handler.

The result is as follows:

BenchmarkOTLPWriteHandler
BenchmarkOTLPWriteHandler/json_with_no_compression
BenchmarkOTLPWriteHandler/json_with_no_compression-10         	   57388	     19514 ns/op	   21898 B/op	     355 allocs/op
BenchmarkOTLPWriteHandler/json_with_gzip
BenchmarkOTLPWriteHandler/json_with_gzip-10                   	   36260	     33121 ns/op	   68106 B/op	     365 allocs/op
BenchmarkOTLPWriteHandler/proto_with_no_compression
BenchmarkOTLPWriteHandler/proto_with_no_compression-10        	   77604	     15191 ns/op	   19700 B/op	     263 allocs/op
BenchmarkOTLPWriteHandler/proto_with_gzip
BenchmarkOTLPWriteHandler/proto_with_gzip-10                  	   49032	     24456 ns/op	   64703 B/op	     270 allocs/op
PASS

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[friedrichg]

CI doesn't run this. I am curious what are the numbers you are getting. do you mind sharing as a comment?

[SungJin1212]

@friedrichg
Sure, I add it to the PR description.

[CharlieTLe]

Could we reuse some of the setup and timing parts of each test type to keep the code DRY?

[SungJin1212]

@CharlieTLe
I have refactored the test code!

[SungJin1212]

@CharlieTLe
Could you take a look when you have time?

[alanprot]

Thanks

[CharlieTLe]

I was also able to reproduce a similar benchmark

goos: darwin
goarch: arm64
pkg: github.com/cortexproject/cortex/pkg/util/push
cpu: Apple M1 Pro
BenchmarkOTLPWriteHandler
BenchmarkOTLPWriteHandler/json_with_no_compression
BenchmarkOTLPWriteHandler/json_with_no_compression-10         	   58062	     19842 ns/op	   21834 B/op	     353 allocs/op
BenchmarkOTLPWriteHandler/json_with_gzip
BenchmarkOTLPWriteHandler/json_with_gzip-10                   	   35395	     33751 ns/op	   68043 B/op	     363 allocs/op
BenchmarkOTLPWriteHandler/proto_with_no_compression
BenchmarkOTLPWriteHandler/proto_with_no_compression-10        	   75156	     15652 ns/op	   19636 B/op	     261 allocs/op
BenchmarkOTLPWriteHandler/proto_with_gzip
BenchmarkOTLPWriteHandler/proto_with_gzip-10                  	   46663	     26699 ns/op	   64640 B/op	     268 allocs/op
PASS