[TEST] otlp grpc exporter retry test fix

[dbarker]

Fixes #3310

Changes

The main thread of the grpc retry test now waits on a future until the grpc server is ready.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[netlify]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.52%. Comparing base (2566fb6) to head (d0c0908).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3311      +/-   ##
==========================================
+ Coverage   89.50%   89.52%   +0.02%     
==========================================
  Files         210      210              
  Lines        6522     6522              
==========================================
+ Hits         5837     5838       +1     
+ Misses        685      684       -1     

see 1 file with indirect coverage changes

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[marcalff]

LGTM, thanks for the fix.