feat(observability): introduce minimal tracing implementation

[diegomarquezp]

Summary

This PR introduces a new tracing mechanism in GAX that allows recording traces using OpenTelemetry. It provides a way of recording spans and attributes, following the existing ApiTracer class pattern with a few tracing-specific additions. The implementation is meant to be extensible to support other implementations.

New Classes

• TraceManager: An interface for managing spans and attributes; can be implemented by observability frameworks.
• OpenTelemetryTraceManager: An implementation of TraceManager that uses the OpenTelemetry API.
• AppCentricTracer: An ApiTracer implementation that delegates span management to a TraceManager.
• AppCentricTracerFactory: A factory for creating AppCentricTracer instances.
• ApiTracerContext: A context object that carries information (like EndpointContext's server address property) used to infer common attributes for all tracers.
• Span: A handle returned by TraceManager to manage the lifecycle of a specific span (ending it, recording errors, or setting attributes).

Approach

Connecting Tracer with Manager

The implementation aims to decouple AppCentricTracer from TraceManager. When a tracer starts an operation or an attempt, it requests a Span from the recorder. This handle allows the tracer to update the span (e.g., adding attributes or recording errors) to keep AppCentricTracer separated from specific recorder implementations (like OpenTelemetry's Span object).

Attribute Inference via ApiTracerContext

To provide a source of Span Attributes that are common to all operations, we introduced ApiTracerContext. This context is passed to ApiTracerFactory and contains information such as serverAddress (provided by EndpointContext). It is operated by ClientContext.
Initially, only serverAddress is contained in this class and it's meant to obtain the server.address attribute.
The class is ultimately operated by AppCentricTracer to extract the necessary attributes.

Integration Tests

A new integration test, ITOtelTracing, was added to the java-showcase module:

• It validates that the expected spans (operation and attempt spans) are recorded with the correct names, parent-child relationships, and attributes (including server.address and gcp.client.language).

Note on java-bigtable downstream check

Since SkipTrailersTest mocks the tracer factory, the EndpointContext call to apiTracerFactory.withContext() returns a null factory, causing a null pointer exception when building the client context.
We expect the test to be adjusted with this change with the next release.

Confirmation in Cloud Trace