fix(auth): remove singleton and restore normal usage of otelgrpc.clientHandler

[quartzmo]

refs: googleapis/google-api-go-client#2321
refs: googleapis/google-api-go-client#2329
refs: open-telemetry/opentelemetry-go-contrib#4226

[gemini-code-assist]

Summary of Changes

Hello @quartzmo, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refactors the gRPC transport layer's OpenTelemetry integration by removing a temporary singleton workaround for the otelgrpc.clientHandler. This workaround was initially put in place to mitigate a known memory leak. With this change, the system now directly instantiates the client handler, simplifying the code and indicating that the underlying issue in the OpenTelemetry library has been resolved or is no longer a concern. The overall impact is a cleaner, more standard implementation of OpenTelemetry tracing for gRPC calls.

Highlights

• Singleton Removal: The singleton pattern previously used for otelgrpc.clientHandler has been removed, reverting to normal instantiation.
• Workaround Elimination: This change eliminates a workaround that was implemented to address a memory leak issue in the opentelemetry-go-contrib library, indicating the underlying problem has likely been resolved.
• Code Simplification: Related imports for sync and google.golang.org/grpc/stats, which were only necessary for the singleton implementation, have been removed, simplifying the codebase.

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[gemini-code-assist]

Code Review

This pull request removes a workaround for a memory leak in otelgrpc.NewClientHandler that has since been fixed in the upstream OpenTelemetry library. The change correctly removes the singleton pattern used to share a stats.Handler across all gRPC connections and restores the standard usage of creating a new otelgrpc.ClientHandler for each connection. This simplifies the code by removing the now-unnecessary singleton logic, including related imports, variables, and the helper function. The changes are clean, correct, and improve the maintainability of the code.