feat(core): add request hook to enrich T4 network spans with GCP resource attributes (E)

[chalmerlowe]

Summary

This PR implements OpenTelemetry tracing in google-api-core:

• Adds an OpenTelemetry client request_hook to otel_grpc.client_interceptor and otel_grpc.aio_client_interceptors.
• Dynamically extracts semantic Google Cloud resource attributes from gRPC request objects:
  • gcp.resource.name
  • gcp.resource.parent
  • gcp.project_id (parsed from standard AIP-122 resource names and parents)
• Injects these attributes directly into the active wire-level span.
• Adds comprehensive unit tests validating extraction from requests and attachment to spans.

Why

Enriches raw gRPC transport spans with high-level Google Cloud resource context without requiring modifications to generated client methods or waiting for higher-level method decorators.

[gemini-code-assist]

Code Review

This pull request introduces OpenTelemetry request hooks to extract and inject Google Cloud semantic and resource attributes (such as resource name, parent, and project ID) from gRPC request objects into OpenTelemetry spans. Specifically, it adds _extract_t4_attributes and _client_request_hook helper functions in _observability.py, registers the hook in the gRPC client interceptors, and includes comprehensive unit tests to verify this behavior. There are no review comments, so I have no feedback to provide.