Document/add concurrency-safety to Console and OTLP exporters' `ForceFlush`/`Shutdown`

Spec requirement

Each LogRecordExporter implementation MUST document its concurrency characteristics (sdk.md:612-613 (permalink)); ForceFlush and Shutdown MUST be safe for concurrent calls (sdk.md:712-713 (permalink)).

Current behavior

logs_sdk/lib/opentelemetry/sdk/logs/export/console_log_record_exporter.rb:27-34 (permalink) (#force_flush/#shutdown) mutate @stopped with no synchronization. exporter/otlp-logs/lib/opentelemetry/exporter/otlp/logs/logs_exporter.rb:108-112 (permalink) (#shutdown) mutates @shutdown and calls @http.finish with no mutex either. Only InMemoryLogRecordExporter (export/in_memory_log_record_exporter.rb:83-99) (permalink) guards this with @mutex.

Suggested fix

Add a mutex around @stopped/@shutdown mutation (and any shared @http access) in both ConsoleLogRecordExporter and the OTLP LogsExporter, and document the concurrency guarantee in each class's description.

Related rows in SPEC_COMPLIANCE_LOGS.md

SDK-42, SDK-50

Start with logs_sdk/lib/opentelemetry/sdk/logs/export/console_log_record_exporter.rb and exporter/otlp-logs/lib/opentelemetry/exporter/otlp/logs/logs_exporter.rb, then compare the mutex usage in export/in_memory_log_record_exporter.rb. Review ForceFlush and Shutdown, including shared @http access in the OTLP exporter. Done means both exporters synchronize the relevant state and document their concurrency guarantees.