[exporterhelper] Fix small batch due to scheduling in batch sender

[carsonip]

Description

• Correctly keep track of bs.activeRequests, which denotes the number of send waiting for next sender in chain to return. This is already done before this PR, but in a way that is vulnerable to unfavorable goroutine scheduling
• Decrease bs.activeRequests by the number of requests blocked by an activeBatch at once, so that it workarounds the "bug" mentioned in (1).

Link to tracking issue

Fixes #9952

Testing

Added unit test

Documentation

[carsonip]

Blocked by #10315, otherwise test will be flaky.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.38%. Comparing base (964e3a9) to head (c456dac).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10337   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         387      387           
  Lines       18308    18312    +4     
=======================================
+ Hits        16913    16917    +4     
  Misses       1049     1049           
  Partials      346      346           

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