Include aggregation temporality in LoggingMetricExporter toString

[thswlsqls]

Fixes #8622

Description

• LoggingMetricExporter.toString() returned the constant "LoggingMetricExporter{}", so create(DELTA) and the default create() (CUMULATIVE) printed identically, hiding this exporter's only setting.
• This reaches real debug output: OpenTelemetrySdk.toString() prints the meter provider, and PeriodicMetricReader.toString() appends "exporter=" + exporter.
• The other three MetricExporter implementations that override toString() (OtlpGrpcMetricExporter, OtlpHttpMetricExporter, OtlpStdoutMetricExporter) already print their temporality.
• Plain concatenation rather than StringJoiner, since there is a single field — the form used elsewhere for single-field toString() (e.g. SdkMeter, SdkObservableInstrument).
• Precedent: Add missing fields to OTLP metric exporters #6402 "Add missing fields to OTLP metric exporters" fixed the same gap for the OTLP metric exporters and left this class out.

return "LoggingMetricExporter{aggregationTemporality=" + aggregationTemporality + "}";

Testing done

• Extended LoggingMetricExporterTest#stringRepresentation to assert that create() prints CUMULATIVE and create(DELTA) prints DELTA; it fails against the old constant.
• ./gradlew :exporters:logging:check --rerun-tasks — 15 tests passed.
• No public API change (toString() signature unchanged), so no apidiff update.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.60%. Comparing base (63ebd94) to head (9945128).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8623      +/-   ##
============================================
- Coverage     91.63%   91.60%   -0.03%     
- Complexity    10326    10343      +17     
============================================
  Files          1013     1013              
  Lines         27295    27352      +57     
  Branches       3205     3215      +10     
============================================
+ Hits          25011    25057      +46     
- Misses         1558     1567       +9     
- Partials        726      728       +2     

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