[exporterhelper] Add exporter_dropped_* reporting capability and metrcs

[bogdan-st]

Description

Add exporter_dropped_* metrics to track dropped items, allowing exporter devs to report back the number of dropped items.

Link to tracking issue

Fixes #13643

Documentation

Added instructions for exporter devs

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[codecov]

Codecov Report

❌ Patch coverage is 75.51020% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.34%. Comparing base (f423e20) to head (b7d8037).

Files with missing lines	Patch %	Lines
...orter/exporterhelper/internal/obs_report_sender.go	36.84%	23 Missing and 1 partial ⚠️

❌ Your patch check has failed because the patch coverage (75.51%) 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   #13704      +/-   ##
==========================================
- Coverage   91.39%   91.34%   -0.06%     
==========================================
  Files         648      648              
  Lines       42626    42719      +93     
==========================================
+ Hits        38959    39020      +61     
- Misses       2846     2875      +29     
- Partials      821      824       +3     

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