[exporterhelper] Switch request context encoding from map to KV list

[dmitryax]

Resolves #13188 (review)

This also required moving to gogoproto to reuse existing opentelemetry.proto.common.v1.KeyValue message.

Benchmarks:

goos: darwin
goarch: arm64
pkg: go.opentelemetry.io/collector/pdata/xpdata/request
cpu: Apple M1 Max

before:
BenchmarkEncodeDecodeContext-10    	  519790	      2278 ns/op	    1376 B/op	      22 allocs/op

after:
BenchmarkEncodeDecodeContext-10    	  747158	      1600 ns/op	     661 B/op	      18 allocs/op

[dmitryax]

No changelog item needed since it's not released yet

[codecov]

Codecov Report

Attention: Patch coverage is 78.12500% with 7 lines in your changes missing coverage. Please review.

Project coverage is 91.53%. Comparing base (d77b601) to head (0689fd0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pdata/xpdata/request/context.go	66.66%	7 Missing ⚠️

❌ Your patch status has failed because the patch coverage (78.12%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13216      +/-   ##
==========================================
- Coverage   91.55%   91.53%   -0.02%     
==========================================
  Files         522      522              
  Lines       28938    28955      +17     
==========================================
+ Hits        26494    26504      +10     
- Misses       1927     1934       +7     
  Partials      517      517              

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