Fix OtlpGrpcProfileExporter toString class name

[thswlsqls]

Description

• OtlpGrpcProfileExporter.toString() used the prefix OtlpGrpcProfilesExporter{ (plural "Profiles"), which does not match the class name. Changed the StringJoiner literal to OtlpGrpcProfileExporter{.
• The sibling gRPC exporters (OtlpGrpcSpanExporter, OtlpGrpcMetricExporter, OtlpGrpcLogRecordExporter) already use their own class name as the prefix; this restores that consistency.
• The shared AbstractGrpcTelemetryExporterTest.stringRepresentation() regex OtlpGrpc[a-zA-Z]*Exporter\{ matches both singular and plural, so it never caught this; hence a profiles-specific assertion.
• Signature unchanged (no apidiff); alpha-module diagnostic string, not user-facing (no CHANGELOG).

Testing done

• Added OtlpGrpcProfileExporterTest#stringRepresentationClassName asserting toString() starts with OtlpGrpcProfileExporter{. Fails before the fix (prefix was OtlpGrpcProfiles...), passes after.
• ./gradlew :exporters:otlp:profiles:check passed, including jApiCmp.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.77%. Comparing base (824334c) to head (52ec970).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8492   +/-   ##
=========================================
  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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