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/core/include/cdm_engine.h

@@ -12,6 +12,7 @@
 #include "file_store.h"
 #include "initialization_data.h"
 #include "lock.h"
+#include "metrics_collections.h"
 #include "oemcrypto_adapter.h"
 #include "scoped_ptr.h"
 #include "timer_metric.h"
@@ -241,7 +242,7 @@ class CdmEngine {
   // dead lock.
   virtual void OnTimerEvent();
 
-  virtual metrics::MetricsGroup* GetMetrics() { return &metrics_; }
+  virtual metrics::EngineMetrics* GetMetrics() { return &metrics_; }
 
  private:
   // private methods
@@ -273,7 +274,7 @@ class CdmEngine {
    * ensure that all data has been properly recorded in the group before
    * it is published.
    */
-  metrics::MetricsGroup metrics_;
+  metrics::EngineMetrics metrics_;
   metrics::TimerMetric life_span_;
 
   CdmSessionMap sessions_;