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/metrics/include/metrics_collections.h

@@ -51,7 +51,7 @@
 //       sts);
 #define M_TIME(CALL, GROUP, METRIC, ...)                 \
   if (GROUP) {                                           \
-    wvcdm::metrics::TimerMetric timer;                   \
+    wvcdm::metrics::Timer timer;                         \
     timer.Start();                                       \
     CALL;                                                \
     (GROUP)->METRIC.Record(timer.AsUs(), ##__VA_ARGS__); \
@@ -488,7 +488,7 @@ class EngineMetrics {
   std::vector<std::shared_ptr<metrics::SessionMetrics>>
       completed_session_metrics_list_;
   // This is used to populate the engine lifespan metric
-  metrics::TimerMetric life_span_internal_;
+  metrics::Timer life_span_internal_;
   CryptoMetrics crypto_metrics_;
   std::string app_package_name_;