Experiment(secret-manager): add tracing transport logic to google-cloud-secret-manager

[chalmerlowe]

Problem

Client libraries currently lack built-in support for OpenTelemetry tracing interceptors. We need a flexible, explicit mechanism to inject these interceptors into the transport pipeline without tying the core low-level helpers too tightly to specific observability features.

Note

This PR is NOT intended to be merged. It is a proof-of-concept intended to inform the design and implementation of changes that need to be made in the GAPIC Generator templates.

Solution

This PR implements explicit interceptor injection in the SecretManagerServiceClient and its gRPC transport.

• Updated SecretManagerServiceClient to resolve OpenTelemetry interceptors using google-api-core helpers and explicitly pass them to the transport.
• Updated SecretManagerServiceGrpcTransport to accept an interceptors list and explicitly apply them to the gRPC channel using grpc.intercept_channel.
• Added unit tests (test_secret_manager_service_client_otel_interceptor_injection and *_disabled variant) to verify that interceptors are correctly injected and applied.

Notes to Reviewers

• Async Deferred: As per the design plan, Async gRPC updates are deferred to a separate PR to keep this implementation small and clean.
• Stacked PRs: This work builds on the foundational helpers introduced in the google-api-core PR (Phase 1).
• Explicit Injection: This demonstrates the "active orchestrator" role of the generated client, delegating resolution to core helpers but explicitly controlling the pipeline.

Fixes #18139 (partially, this is Phase 2 of the larger effort)

[gemini-code-assist]

Code Review

This pull request introduces OpenTelemetry (OTel) tracing support by adding a helper module _otel_helpers.py to resolve and instantiate OTel gRPC interceptors, extending ClientOptions to accept a tracer_provider, and updating the Secret Manager gRPC transport to inject these interceptors. The review feedback highlights Python compatibility issues in _otel_helpers.py due to the use of the | union operator, which is unsupported in older Python versions, and suggests using typing.Union instead. Additionally, it recommends adding a defensive None check for client_options to avoid calling getattr on a None object.

[chalmerlowe]

Warning

Do not merge label added because this is a prototype implementation to discover the needed edits to the GAPIC generator templates. This is not intended to be merged.