log request start when tracing classes

When tracing rest calls for debugging purposes, logging the start
of the request is also extremely helpful, as it makes it possible
to discover that the key issue is glance is in a hung state and
not responding to any requests.

Prereq for blueprint:javelin2

Change-Id: I9aca99119fc1569c78788b11a8a32d72eaecd0a1
Merge "log request start when tracing classes"