Updated metric Distribution and Timer utils.

[ 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

libwvdrmengine/cdm/core/include/crypto_session.h

@@ -479,7 +479,7 @@ class CryptoSession {
   };
 
   metrics::CryptoMetrics* metrics_;
-  metrics::TimerMetric life_span_;
+  metrics::Timer life_span_;
   uint32_t system_id_;
 
   bool open_;