test(o11y): add spanner e2e trace context propagation test

[haphungw]

Execute live Google Cloud Trace pipeline proofs using the google-cloud-spanner dependency.
Expose the needed Spanner client methods as pub so the test can compile.

How to Run This Test Manually

If you want to verify that client-side trace spans correctly parent the Spanner backend spans, you can run this test locally against your own GCP project:

1. Enable the required services:

gcloud services enable spanner.googleapis.com cloudtrace.googleapis.com telemetry.googleapis.com

2. Provision a test Spanner instance and database:

gcloud spanner instances create trace-propagation-test-instance --config=regional-us-central1 --description="Test Instance" --nodes=1
gcloud spanner databases create test-database --instance=trace-propagation-test-instance

3. Run the targeted O11y test:

export GOOGLE_CLOUD_PROJECT="$(gcloud config get project)"
env \
  GOOGLE_CLOUD_RUST_TEST_SERVICE_ACCOUNT=rust-sdk-test@${GOOGLE_CLOUD_PROJECT}.iam.gserviceaccount.com \
  GOOGLE_CLOUD_RUST_TEST_STORAGE_KMS_RING=us-central1 \
  GOOGLE_CLOUD_PROJECT=${GOOGLE_CLOUD_PROJECT} \
  GOOGLE_CLOUD_SPANNER_TEST_INSTANCE=trace-propagation-test-instance \
  GOOGLE_CLOUD_SPANNER_TEST_DATABASE=test-database \
  RUSTFLAGS="--cfg google_cloud_unstable_tracing" \
cargo test --features run-integration-tests --package integration-tests-o11y --test spanner_tracing -- --nocapture

[codecov]

Codecov Report

❌ Patch coverage is 12.50000% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.53%. Comparing base (bf48d4f) to head (2b6d1cd).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gax-internal/src/observability/grpc_tracing.rs	12.50%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5031      +/-   ##
==========================================
- Coverage   92.66%   92.53%   -0.13%     
==========================================
  Files         226      226              
  Lines        8682     8698      +16     
==========================================
+ Hits         8045     8049       +4     
- Misses        637      649      +12     

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[haphungw]

/gcbrun