Add command metrics to IPA API

This will allow the agent to report timing and counter data for commands
that are run on the agent (starting with Standby commands). It will allow a
properly configured agent to send metrics to a remote or locally packaged
statsd. It is also extensible, so other metrics systems can easily be used.

Change-Id: Ic08d4ff78b6fb614b474b956a32eac352a14262a
(cherry picked from commit 5bb56c35ca4172b88f7c17be98b60e4f1772ec24)