Add class Javadoc to LoggingMetricExporter

[thswlsqls]

Description

• Adds a class-level Javadoc to LoggingMetricExporter, which was the only one of the three sibling logging exporters missing one.
• Matches the existing pattern in LoggingSpanExporter and SystemOutLogRecordExporter.
• The wording reflects the actual export() behavior: every metric is logged at INFO level via java.util.logging.

Testing done

• ./gradlew :exporters:logging:check — 15 tests passed.
• Javadoc-only change with no behavior change, so no new tests were added.
• Signature unchanged, so no apidiff update; not user-visible, so no CHANGELOG entry.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: thswlsqls / name: Eunbin Son (8ac56eb)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.77%. Comparing base (824334c) to head (8ac56eb).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8490   +/-   ##
=========================================
  Coverage     78.77%   78.77%           
  Complexity     8579     8579           
=========================================
  Files          1009     1009           
  Lines         28993    28993           
  Branches       3599     3599           
=========================================
  Hits          22839    22839           
  Misses         5311     5311           
  Partials        843      843           

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[thswlsqls]

/easycla

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