debug logging: racing condition in GrpcLoggingInterceptor.java when a Cloud Spanner client session pool is created

race condition originating from the gax-grpc java library in the [GrpcLoggingInterceptor.java](https://source.corp.google.com/piper///depot/google3/third_party/java_src/gax/gax-grpc/src/main/java/com/google/api/gax/grpc/GrpcLoggingInterceptor.java;l=101;bpv=0;bpt=0;rcl=740874501) file when a Cloud Spanner client session pool is created.
see context b/470591768#comment6.