[exporterhelper] Fix potential deadlock in the batch sender

[dmitryax]

Concurrent handling of the flush timeouts can run into a deadlock when a batch is simultaneously sent by reaching the minimum size and flush timeout. The deadlock can happen on the following lines:

• opentelemetry-collector/exporter/exporterhelper/batch_sender.go

  Line 131 in 115bc8e

  bs.resetTimerCh <- struct{}{}

• opentelemetry-collector/exporter/exporterhelper/batch_sender.go

  Line 87 in 115bc8e

  bs.mu.Lock()

[dmitryax]

@carsonip PTAL

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.57%. Comparing base (760f773) to head (4ca2db6).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10315      +/-   ##
==========================================
- Coverage   92.58%   92.57%   -0.01%     
==========================================
  Files         387      387              
  Lines       18254    18250       -4     
==========================================
- Hits        16900    16895       -5     
- Misses       1008     1009       +1     
  Partials      346      346              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[carsonip]

bugfix lgtm, I've managed to reproduce the deadlock with an updated test (see code suggestion)

[dmitryax]

Thanks @carsonip!