impl(v2): add client request spans to gax.Invoke

[westarle]

This PR implements T3 spans (Client Request) in gax.Invoke when the TRACING feature flag is enabled.

Key changes:

• Added StartClientRequestSpan and EndClientRequestSpan in telemetry.go to handle the lifecycle of T3 spans.
• Hooked these into Invoke using a closure pattern to avoid leaking OpenTelemetry types into invoke.go.
• The T3 span name is read from the context (key: client_span_name) injected by the generator, falling back to rpc_method if missing.
• Populates error attributes on the span using the existing ExtractTelemetryErrorInfo helper.

Testing:

• Added a new test suite invoke_tracing_test.go that uses OTel's tracetest to verify that spans are correctly created and populated with attributes. All tests in v2 passed.