Adds first-byte latency timings for GET requests.

This was an oustanding TODO for StatsD Swift metrics.  A new timing
metric is tracked for (only) GET requests for accounts, containers,
and objects:

  proxy-server.<req_type>.GET.<status_int>.first-byte.timing

Also updated StatsD documentation in the Admin Guide to clarify that
timing metrics are sent in units of milliseconds.

Change-Id: I5bb781c06cefcb5280f4fb1112a526c029fe0c20
Merge "Adds first-byte latency timings for GET requests."