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/src/timer_metric.cpp

@@ -5,19 +5,26 @@
 
 namespace wvcdm {
 namespace metrics {
-void TimerMetric::Start() {
-  start_ = clock_.now();
+using ClockType = std::chrono::steady_clock;
+using TimePoint = std::chrono::time_point<ClockType>;
+
+void Timer::Start() {
+  start_ = ClockType::now();
   is_started_ = true;
 }
 
-void TimerMetric::Clear() { is_started_ = false; }
+void Timer::Clear() { is_started_ = false; }
 
-double TimerMetric::AsMs() const {
-  return (clock_.now() - start_) / std::chrono::milliseconds(1);
+double Timer::AsMs() const {
+  if (!is_started_) return 0.0;
+  const TimePoint end = ClockType::now();
+  return (end - start_) / std::chrono::milliseconds(1);
 }
 
-double TimerMetric::AsUs() const {
-  return (clock_.now() - start_) / std::chrono::microseconds(1);
+double Timer::AsUs() const {
+  if (!is_started_) return 0.0;
+  const TimePoint end = ClockType::now();
+  return (end - start_) / std::chrono::microseconds(1);
 }
 }  // namespace metrics
 }  // namespace wvcdm