Fix PeriodicMetricReader shutdown race losing final metrics

[trask]

When a periodic export is in-flight during shutdown, doRun() sees exportAvailable=false and drops the final collection. Then exporter.shutdown() cancels the in-flight export via cancelAll().

Fix: wait for any in-flight flush to complete before the final doRun() in shutdown(). Track the in-flight flush via a volatile flushInProgress field set at the start of each doRun() cycle.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.26%. Comparing base (ca41b47) to head (5d499ee).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8299   +/-   ##
=========================================
  Coverage     90.25%   90.26%           
- Complexity     7683     7684    +1     
=========================================
  Files           850      850           
  Lines         23190    23193    +3     
  Branches       2353     2353           
=========================================
+ Hits          20930    20935    +5     
+ Misses         1533     1532    -1     
+ Partials        727      726    -1     

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[jack-berg]

Thanks!