[chore] Update exporterhelper README.md

[dmitryax]

Fix sending_queue docs: the comment for queue_size was cut from the section by blocking item

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.46%. Comparing base (2493b6e) to head (cfceb4f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12364      +/-   ##
==========================================
+ Coverage   91.42%   91.46%   +0.04%     
==========================================
  Files         467      468       +1     
  Lines       25609    25662      +53     
==========================================
+ Hits        23414    23473      +59     
+ Misses       1789     1783       -6     
  Partials      406      406              

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