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/include/timer_metric.h

@@ -9,13 +9,23 @@ namespace metrics {
 class TimerMetric {
 
  public:
+  // Starts the clock running. If the clock was previously set, this resets it.
+  // IsStarted will return true after this call.
   void Start();
+  // Returns whether or not the timer has started.
+  bool IsStarted() const { return is_started_; };
+  // Stops the clock and clears the current value.  IsStarted will return false
+  // after this call.
+  void Clear();
+  // Returns the current clock value as milliseconds (AsMs) or microseconds
+  // (AsUs).
   double AsMs() const;
   double AsUs() const;
 
  private:
   double sec_;
   double usec_;
+  bool is_started_;
 
 };