auth: Clean up OpenTelemetry keys in tests

[westarle]

To make sure we are recording the right thing and make it easy to review changes, I would like to use specific strings instead of constants in tests.

For example in grpctransport_otel_test.go there is a mix:

https://github.com/westarle/google-cloud-go/blob/a3a4a8f1a4bceb096d27e1d3d2ec3e06af599cf9/auth/grpctransport/grpctransport_otel_test.go#L241-L251

This cleanup task will make sure the attribute strings themselves are visible in the tests.