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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.94%. Comparing base (104d56e) to head (2bc10e3).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8525      +/-   ##
============================================
+ Coverage     90.93%   90.94%   +0.01%     
  Complexity    10209    10209              
============================================
  Files          1013     1013              
  Lines         27175    27175              
  Branches       3184     3184              
============================================
+ Hits          24712    24715       +3     
+ Misses         1735     1733       -2     
+ Partials        728      727       -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.

[jack-berg]

Meh I don't love the "Dropping metrics" because its its not true - metrics are actively being exported! I know you're just continuing the existing text used in the log, but now that its an exception that callers might key off of I want to improve the language. Maybe something like: "Export is already in progress, skipping flush". Would want to update the log line on 295 to match.

Also, extract this message to a constant so it can be used across this exception, the one line line 296, and the log line on 295.

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