[exporterhelper] New exporter helper for custom requests

[dmitryax]

Introduce a new exporter helper that operates over client-provided requests instead of pdata. The helper user now has to provide Converter - an interface with a function implementing translation of pdata Metrics/Traces/Logs into a user-defined Request. Request is an interface with only one required function Export.

It opens a door for moving batching to the exporter, where batches will be built from client data format, instead of pdata. The batches can be properly sized by custom request size, which can be different from OTLP. The same custom request sizing will be applied to the sending queue. It will also improve the performance of the sending queue retries for non-OTLP exporters, they don't need to translate pdata on every retry.

This is an implementation alternative to #7874 as suggested in #7874 (comment)

Tracking Issue: #8122

[codecov]

Codecov Report

Patch coverage: 82.24% and project coverage change: -0.12% ⚠️

Comparison is base (ebed8dd) 90.27% compared to head (0627049) 90.16%.
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8178      +/-   ##
==========================================
- Coverage   90.27%   90.16%   -0.12%     
==========================================
  Files         301      302       +1     
  Lines       15596    15733     +137     
==========================================
+ Hits        14080    14186     +106     
- Misses       1227     1254      +27     
- Partials      289      293       +4     

Files Changed	Coverage Δ
exporter/exporterhelper/request.go	40.00% <40.00%> (ø)
exporter/exporterhelper/queued_retry.go	90.39% <65.38%> (-4.87%)	⬇️
exporter/exporterhelper/logs.go	83.87% <85.71%> (+3.22%)	⬆️
exporter/exporterhelper/metrics.go	83.87% <85.71%> (+3.22%)	⬆️
exporter/exporterhelper/traces.go	84.04% <85.71%> (+3.09%)	⬆️
exporter/exporterhelper/common.go	91.30% <100.00%> (+0.49%)	⬆️
...porter/exporterhelper/internal/persistent_queue.go	100.00% <100.00%> (ø)
...rter/exporterhelper/internal/persistent_storage.go	89.57% <100.00%> (-3.07%)	⬇️
...xporterhelper/internal/persistent_storage_batch.go	85.36% <100.00%> (ø)

... and 3 files with indirect coverage changes

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[codeboten]

The change looks ok, just one question and a suggestion

[jpkrohling]

Looks nice, I prefer this over the previous PR.

[dmitryax]

@codeboten, how did it happen to be added to 0.83.0? It should be part of 0.84.0, right?

[codeboten]

@dmitryax yeah sorry i tagged it incorrectly.... let me revisit the milestone labeling i did manually yesterday