Tracing for auth components

[dbolduc]

All clients have the option to enable tracing

https://github.com/googleapis/google-cloud-rust/blob/33b8ab7d0e0937177b3b6c826bcb4c1e0da8dec9/src/gax/src/options/mod.rs#L231

But there is no visibility into what the auth layer is doing. The auth layer may send RPCs which will render as an unexplained gap in a waterfall chart.

We will want to instrument the auth layer for traces, and pass along the tracing configuration from gax clients to the credential types.