Send API timing to statsd for all requests, not just writes

Send API timing to statsd for all requests, not just writes

The rate will be about 8k req/s, whereas currently graphite1001
receives about 350k metric updates per second, so we would expect
this to increase statsd load by about 2%.

This would have been very helpful in diagnosing API cluster overload
incidents such as the one on July 11.

Change-Id: I0a248e1bcec05915d98ec735f4cfb9800e64412d