Use failExceptionally in PeriodicMetricReader when exporter is busy

[vivekp14]

Fixes #8433

When forceFlush() is called while a periodic export is already in
progress, PeriodicMetricReader was silently failing with plain fail().

Updated to use failExceptionally(IllegalStateException) so callers
can inspect the cause via CompletableResultCode#getFailureThrowable().

Also added JavaDoc to forceFlush() documenting this edge case.

[linux-foundation-easycla]

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• ✅ login: vivekp14 / name: vivekp14 (d4511d3)

[kropptrevor]

As it says next to the spotless violations:

Run './gradlew spotlessApply' to fix all violations.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.51%. Comparing base (a5cd87f) to head (34fee7e).
⚠️ Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
...metry/sdk/metrics/export/PeriodicMetricReader.java	50.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (50.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8525      +/-   ##
============================================
- Coverage     78.54%   78.51%   -0.03%     
- Complexity     8468     8602     +134     
============================================
  Files          1008     1013       +5     
  Lines         28824    29148     +324     
  Branches       3569     3631      +62     
============================================
+ Hits          22639    22887     +248     
- Misses         5342     5419      +77     
+ Partials        843      842       -1     

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[vivekp14]

Build is now passing. I noticed the patch coverage is at 50% — happy to add a test covering the forceFlush failure path if that would help get this merged.

[kropptrevor]

@vivekp14 Yes, please add/modify tests. Make sure to test the tests by breaking it to see the test fail.