Caching of auth client regressed by #705

[ofrobots]

I believe that PR #705 regressed the earlier fix to cache the auth client I contributed in #661. This means that a new cache client is created on each API request we make, adding an unnecessary performance overhead. Here's some Cloud Trace output I get:

Note the extra 141ms added to this request, and every other request.

The cause is that the function makeAuthorizedRequest now passes a brand new object to util.makeRequest each time. We cache the authClient on that object, which is temporary and not available the next time through.

Would it be possible add a test for this to make sure this doesn't get regressed?

[stephenplusplus]

Good catch, sorry to introduce the regression. This is a good time to incorporate https://github.com/stephenplusplus/google-auto-auth into gcloud, which will cache the client for us.

[ofrobots]

Ooh, google-auto-auth looks interesting. Let me see if we can use it in @google/cloud-trace.