[OpenTelemetry] Default open telemetry configuration behind experiment

[stankiewicz]

Add default open telemetry tracing configuration for dataflow runners.
no-op - change is behind experiment.

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[gemini-code-assist]

Summary of Changes

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This pull request introduces default OpenTelemetry tracing properties for Dataflow runners. By injecting standard OTLP exporter settings and service metadata into the pipeline options, it enables easier observability integration for users who opt-in via the experimental flag.

Highlights

• OpenTelemetry Configuration: Added default OpenTelemetry tracing configuration for Dataflow runners, encapsulated behind an experimental flag.
• Feature Flagging: The new configuration is gated by the 'enable_opentelemetry_tracing' experiment, ensuring no impact on existing production workloads.

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Code Review

This pull request adds support for configuring OpenTelemetry tracing in the DataflowRunner when the 'enable_opentelemetry_tracing' experiment is enabled. It retrieves and updates the OpenTelemetry properties on SdkHarnessOptions. A review comment correctly identifies a potential issue where the retrieved properties map might be immutable, which would lead to an UnsupportedOperationException when modified. The reviewer suggested copying the existing properties into a new mutable HashMap to avoid this error.

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R: @tvalentyn added as fallback since no labels match configuration

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