[OpenTelemetry] Add tracing support to GCP Spanner IO

[stankiewicz]

This PR introduces OpenTelemetry (OTel) support for Google Cloud Spanner IO, enabling better observability, tracing, and metrics for Spanner operations within Beam pipelines.

• Configuration: Added withEnableOpenTelemetryTracing() to SpannerConfig and SpannerIO (Write and ReadChangeStream) to allow users to easily toggle OTel tracing.

• Client Initialization: Updated SpannerAccessor and its underlying SpannerOptions builder to accept an OpenTelemetry instance. When enabled, this automatically configures extended tracing, API tracing, and end-to-end tracing, while swapping OpenCensus for OpenTelemetry metrics/traces in the Spanner client.

• Pipeline Options Integration: Updated the @setup lifecycle methods across various DoFns (e.g., BatchSpannerRead, NaiveSpannerRead, CreateTransactionFn, and Change Stream DoFns) to extract the OpenTelemetry instance directly from PipelineOptions via SdkHarnessOptions.

• Change Streams DAO: Updated DaoFactory to hold and pass the OpenTelemetry instance to the Spanner metadata and admin clients.

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Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the observability of Google Cloud Spanner IO within Apache Beam by integrating OpenTelemetry. By enabling users to configure OTel tracing and metrics, it provides a more modern and standardized approach to monitoring Spanner operations. The changes involve updating the Spanner client initialization, configuring pipeline options, and ensuring that all relevant DoFns and DAOs are aware of the OpenTelemetry instance.

Highlights

• OpenTelemetry Integration: Added comprehensive OpenTelemetry (OTel) support to Google Cloud Spanner IO, allowing for improved observability, tracing, and metrics within Beam pipelines.
• Configuration Toggles: Introduced withEnableOpenTelemetryTracing() to SpannerConfig and SpannerIO, enabling users to toggle OTel tracing for Read and Write operations.
• Client Initialization: Updated SpannerAccessor to accept an OpenTelemetry instance, enabling the configuration of extended, API, and end-to-end tracing while migrating from OpenCensus to OpenTelemetry.
• Lifecycle Management: Updated @setup methods across various DoFns to inject the OpenTelemetry instance directly from PipelineOptions, ensuring consistent observability across the pipeline.

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Code Review

This pull request integrates OpenTelemetry tracing and metrics support into the Beam Google Cloud Spanner IO connector. It updates various DoFn setup methods to retrieve OpenTelemetry from PipelineOptions and configures SpannerAccessor and SpannerConfig to enable OpenTelemetry tracing when specified. The review feedback highlights two issues: a potential IllegalStateException or NullPointerException in SpannerAccessor when calling get() on a ValueProvider without checking isAccessible() or handling nulls, and a malformed Javadoc comment in ReadChangeStreamPartitionDoFn containing nested {@link} tags.

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