Add retry dropped item metrics and an exhausted retry error marker for exporter helper retries

[iblancasa]

Description

• Add a dedicated experr.NewRetriesExhaustedErr wrapper so exporters can detect when all retry attempts failed
• Record new otelcol_exporter_retry_dropped_{spans,metric_points,log_records} counters when retries are exhausted, alongside existing send-failed metrics

Link to tracking issue

Fixes #13956

[codecov]

Codecov Report

❌ Patch coverage is 98.83721% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.62%. Comparing base (02fbe61) to head (869d3f8).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
exporter/exporterhelper/internal/retry_sender.go	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13957      +/-   ##
==========================================
- Coverage   91.64%   91.62%   -0.02%     
==========================================
  Files         654      654              
  Lines       42701    42788      +87     
==========================================
+ Hits        39133    39205      +72     
- Misses       2751     2762      +11     
- Partials      817      821       +4     

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