Creates a new license request latency metric

Previously, we did not have a license request latency metric. This is a
notable limitation in our metrics. This adds a metric that captures the
timing between a GenerateKeyRequest and an AddKey operation.

Bug: 72994956
Test: New unit tests. Google Play
Change-Id: If99c187399c02f9b5d4c355732af7588bbbefb11

libwvdrmengine/cdm/metrics/src/timer_metric.cpp

@@ -11,6 +11,13 @@ void TimerMetric::Start() {
   gettimeofday(&tv, NULL);
   sec_ = tv.tv_sec;
   usec_ = tv.tv_usec;
+  is_started_ = true;
+}
+
+void TimerMetric::Clear() {
+  is_started_ = false;
+  sec_ = 0;
+  usec_ = 0;
 }
 
 double TimerMetric::AsMs() const {