Refactored metrics to support pull model.

MetricsGroup split into 3 groups, session, engine, and crypto.
MetricsFrontEnd and Report removed.

This is a merge from wvgerrit/28420

Bug: 36217927
Test: Added unit tests to cover modified code.
Change-Id: I2f39f99ce88cc2229d6d1aa9459c67c5b86ccef4

libwvdrmengine/cdm/metrics/include/event_metric.h

@@ -18,7 +18,7 @@
 #include "lock.h"
 
 #include "distribution.h"
-#include "metric_publisher.h"
+#include "metric_serialization.h"
 
 namespace wvcdm {
 namespace metrics {
@@ -27,11 +27,11 @@ class EventMetricTest;
 
 // This base class provides the common defintion used by all templated
 // instances of EventMetric.
-class BaseEventMetric : public MetricPublisher {
+class BaseEventMetric : public MetricSerializable {
  public:
-  // Publish metric values to the MetricNotification. |subscriber| must
+  // Send metric values to the MetricSerializer. |serializer| must
   // not be null and is owned by the caller.
-  virtual void Publish(MetricNotification* subscriber);
+  virtual void Serialize(MetricSerializer* serializer);
 
  protected:
   // Instantiates a BaseEventMetric.