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/certificate_provisioning.h

@@ -6,7 +6,7 @@
 #include <string>
 
 #include "crypto_session.h"
-#include "metrics_group.h"
+#include "metrics_collections.h"
 #include "oemcrypto_adapter.h"
 #include "service_certificate.h"
 #include "wv_cdm_types.h"
@@ -18,7 +18,7 @@ class FileSystem;
 
 class CertificateProvisioning {
  public:
-  CertificateProvisioning(metrics::MetricsGroup* metrics) :
+  CertificateProvisioning(metrics::CryptoMetrics* metrics) :
       crypto_session_(metrics),
       cert_type_(kCertificateWidevine),
       service_certificate_(NULL) {};