Add toString to OtlpJsonLogging exporters

[thswlsqls]

Fixes #8724

Description

• OtlpJsonLoggingMetricExporter, OtlpJsonLoggingSpanExporter and OtlpJsonLoggingLogRecordExporter had no toString(), so they printed an identity hash such as OtlpJsonLoggingMetricExporter@6d06d69c.
• These strings are user-visible: PeriodicMetricReader.toString() embeds the exporter as "exporter=" + exporter.
• For the metric exporter this hid its only user-settable option, the AggregationTemporality passed to create(AggregationTemporality).
• Formats match the siblings in :exporters:logging: LoggingMetricExporter{aggregationTemporality=...}, LoggingSpanExporter{}, SystemOutLogRecordExporter{}.
• Delegating to the wrapped OtlpStdout*Exporter does not work: it is not exposed, and create(AggregationTemporality) never forwards the temporality, so the delegate still reports the builder default alwaysCumulative().
• Same shape as Add toString to OTLP exporters #5686 and Add toString to ComposableAnnotatingSampler #8645; the metric format follows Include aggregation temporality in LoggingMetricExporter toString #8623.

Testing done

• Added stringRepresentation() to OtlpJsonLoggingMetricExporterTest (cumulative and delta), OtlpJsonLoggingSpanExporterTest and OtlpJsonLoggingLogRecordExporterTest. All three fail on main and pass with this change.
• ./gradlew :exporters:logging-otlp:check — 80 tests passed, 0 failures.
• Committed the updated docs/apidiffs/current_vs_latest/opentelemetry-exporter-logging-otlp.txt.

[otelbot]

⚠️ API changes detected — additional maintainer review required

@jack-berg @jkwatson

This PR modifies the public API surface area of the following module(s):

• opentelemetry-exporter-logging-otlp

Please review the changes in docs/apidiffs/current_vs_latest/ carefully before approving.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.26%. Comparing base (f4bb945) to head (98c710b).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8725   +/-   ##
=========================================
  Coverage     91.26%   91.26%           
- Complexity    10461    10464    +3     
=========================================
  Files          1006     1006           
  Lines         28244    28247    +3     
  Branches       3564     3564           
=========================================
+ Hits          25778    25781    +3     
  Misses         1674     1674           
  Partials        792      792           

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