Fix race condition causing flaky PeriodicMetricReaderTest

[jack-berg]

Fixes #8306

Race:

1. forceFlush() calls doRun(), which sets exportAvailable = false and assigns flushInProgress = flushResult
2. shutdown() calls flushInProgress.join() — waiting for the in-flight export
3. The blocking export completes → whenComplete fires: flushResult.succeed() (line 226) → shutdown() unblocks
4. Race: exportAvailable.set(true) (line 227) hasn't run yet
5. shutdown() calls the final doRun() → CAS exportAvailable true→false fails (still false), so the export is dropped

cc @trask

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.28%. Comparing base (5f5b054) to head (5c3619c).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8317      +/-   ##
============================================
+ Coverage     90.25%   90.28%   +0.02%     
- Complexity     7691     7693       +2     
============================================
  Files           850      850              
  Lines         23203    23207       +4     
  Branches       2356     2356              
============================================
+ Hits          20943    20952       +9     
+ Misses         1532     1530       -2     
+ Partials        728      725       -3     

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