This PR introduces feature-gated instrumentation support for gRPC clients generated by Librarian, specifically targeting gRPC-only clients using the grpc-client template set.

For googleapis/google-cloud-rust#3418

Changes:

1. internal/sidekick/internal/rust/templates/grpc-client/transport.rs.mustache:

   • Added a lazy_static definition for INSTRUMENTATION_CLIENT_INFO within the existing info module. This static is conditionally compiled based on the google_cloud_unstable_tracing cfg flag and the Codec.DetailedTracingAttributes Sidekick option.
   • Modified the StorageControl::new function to call .with_instrumentation() on the gaxi::grpc::Client instance (inner). This call is conditional based on gaxi::options::tracing_enabled(&config) at runtime, and the entire block is guarded by {{#Codec.DetailedTracingAttributes}} and #[cfg(google_cloud_unstable_tracing)].

2. internal/sidekick/internal/rust/templates/common/Cargo.toml.mustache:

   • Added [lints] workspace = true to ensure generated crates inherit workspace lint configurations, including the unexpected_cfgs allowance for google_cloud_unstable_tracing. This is also guarded by {{#Codec.DetailedTracingAttributes}}.

Testing Steps:

Test PR: googleapis/google-cloud-rust#3546

The following steps were performed to test these template changes, using the google-cloud-storage crate's StorageControl client as a test case, as it uses the grpc-client templates.

Setup:

• Librarian repo on branch feat/t4-grpc-conditional-instr.
• google-cloud-rust repo on branch feat/t4-grpc-client-with-instr (which contains the with_instrumentation method on gaxi::grpc::Client).

Test Case 1: DetailedTracingAttributes ON

1. Configure: Modify /usr/local/google/home/westarle/src/otel-rust/projects/t4-grpc-network-span/google-cloud-rust/src/storage/src/generated/gapic/.sidekick.toml to ensure detailed-tracing-attributes = true.

2. Regenerate: From the Librarian repo root:

   go run ./cmd/sidekick refresh -project-root /usr/local/google/home/westarle/src/otel-rust/projects/t4-grpc-network-span/google-cloud-rust -output src/storage/src/generated/gapic

3. Format: In the google-cloud-rust repo:

   cargo fmt -p google-cloud-storage

4. Verify Diffs: In the google-cloud-rust repo:

   git diff src/storage/src/generated/gapic/transport.rs

   Confirmed that the INSTRUMENTATION_CLIENT_INFO static and the call to inner.with_instrumentation(Some(&info::INSTRUMENTATION_CLIENT_INFO)) were present and correctly guarded.

5. Test with RUSTFLAGS: In the google-cloud-rust repo:

   RUSTFLAGS="--cfg google_cloud_unstable_tracing" cargo test -p google-cloud-storage
   RUSTFLAGS="--cfg google_cloud_unstable_tracing" cargo clippy -p google-cloud-storage -- -D warnings
   cargo semver-checks check-release -p google-cloud-storage

   All checks passed.

6. Test without RUSTFLAGS: In the google-cloud-rust repo:

   cargo test -p google-cloud-storage
   cargo clippy -p google-cloud-storage -- -D warnings
   cargo semver-checks check-release -p google-cloud-storage

   All checks passed.

Test Case 2: DetailedTracingAttributes OFF

1. Configure: Modify /usr/local/google/home/westarle/src/otel-rust/projects/t4-grpc-network-span/google-cloud-rust/src/storage/src/generated/gapic/.sidekick.toml to set detailed-tracing-attributes = false.
2. Regenerate: Repeat step 3 from Test Case 1.
3. Format: Repeat step 4 from Test Case 1.
4. Verify Diffs: In the google-cloud-rust repo:

   git diff src/storage/src/generated/gapic/transport.rs

   Confirmed that the guarded blocks are now absent in src/storage/src/generated/gapic/transport.rs.
5. Test without RUSTFLAGS: Repeat step 7 from Test Case 1.

   cargo test -p google-cloud-storage
   cargo clippy -p google-cloud-storage -- -D warnings
   cargo semver-checks check-release -p google-cloud-storage

   All checks passed.

Cleanup:

• Restored google-cloud-rust/src/storage/src/generated/gapic/.sidekick.toml to its original state.

These tests confirm that the template changes correctly inject the instrumentation logic only when both the Sidekick option and the compile-time cfg flag are active, and do not affect the build when disabled.