Provide further breakdown of "processing" phase metric (for oauth2 probes)

[nemobis]

We're using the oauth2 module for some probes (see also #1155) and occasionally they're very slow. We're not sure in what phase the slowness is happening, except that it's in the processing phase (probe_http_duration_seconds{phase="processing"}).

It would be nice to (optionally?) provide a further breakdown of the processing phase, or maybe just an additional gauge to measure how long it takes to do the oauth2 handshake.

For a separate gauge, a good point might be the line where NewClientFromConfig is invoked. As for the current durationGaugeVec, it extracts information from httptrace.ClientTrace and I'm not sure that can be broken down further.