[ Merge of http://go/wvgerrit/137811 ]
Renamed TimerMetric to Timer. Timer is used to generate durations
included in metrics, but is not a metric itself. The method of
getting the current time did not require creating an instance of
std::steady_clock.
Updated Distribution and Timer to use default initializers instead of
constructor initialization list.
Bug: 204946540
Test: Metric unit tests
Change-Id: I7ed291b586347dd0b7ab305960883bec04637315
f5759c5149