[Dataflow Java] Support the existing --logMdc option by adding the present mdc values to custom_data json field.

[scwhittle]

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Summary of Changes

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This pull request enhances the logging capabilities of Dataflow Java workers by enabling the inclusion of SLF4J Mapped Diagnostic Context (MDC) values directly within the custom_data field of log entries. This change improves log traceability and debugging by providing more contextual information, activated via an existing command-line option, without altering the core logging mechanism.

Highlights

• MDC Logging Support: Implemented support for the existing --logMdc option in Dataflow Java workers, allowing SLF4J Mapped Diagnostic Context (MDC) values to be included in log messages.
• Log Output Enhancement: MDC key-value pairs are now serialized into a custom_data JSON field within the log entry, providing richer contextual information for debugging and analysis.
• Configuration Integration: The new MDC logging functionality is integrated with DataflowWorkerLoggingInitializer to respect the --logMdc command-line option, enabling or disabling this feature based on job configuration.
• Improved Test Coverage: Added new unit tests to DataflowWorkerLoggingHandlerTest to thoroughly validate the correct inclusion and exclusion of MDC data in log outputs under various scenarios.
• MDC Context Management: Updated existing tests in DataflowWorkerLoggingHandlerTest and BeamFnLoggingClientTest to utilize MDC.putCloseable, ensuring proper cleanup of MDC context after test execution.

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R: @m-trieu for label java.

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[scwhittle]

R: @kennknowles

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