feat(gax): Implement trace context extraction and injection with integration test

[westarle]

This PR implements native trace context propagation for Google Cloud Java clients, enabling App Centric Observability by linking client-side telemetry with server-side resources.

It introduces a new injectTraceContext method to the ApiTracer interface and implements it in SpanTracer using the OpenTelemetry W3CTraceContextPropagator. The extracted trace context is then
dynamically injected into the outgoing request metadata natively within GrpcClientCalls and HttpJsonClientCalls for unary requests. CompositeTracer has also been updated to delegate this new
method to its child tracers.

By pushing this logic down into the ApiTracer and core call factories, we ensure that trace headers (e.g., traceparent) are automatically propagated whenever tracing is enabled, without requiring users to configure separate transport-specific interceptors or rely on external OpenTelemetry instrumentation agents.

Finally, this PR includes an end-to-end integration test (ITOtelTracePropagation) in the java-showcase module, which verifies the successful round-trip of W3C trace headers against the gapic-showcase server.

[gemini-code-assist]

Code Review

This pull request implements OpenTelemetry trace context propagation for gRPC and HTTP/JSON transports by adding an injectTraceContext method to the ApiTracer interface and implementing it in SpanTracer. The GrpcClientCalls and HttpJsonClientCalls classes were updated to inject trace headers into outgoing requests. A review comment identifies a potential issue in the gRPC implementation where direct mutation of the metadata object could lead to side effects during retries, suggesting the creation of a new metadata instance instead.