Introduce a MetricsDataProvider interface.

chrome/browser/metrics/metrics_service.h

Index: chrome/browser/metrics/metrics_service.h
===================================================================
--- chrome/browser/metrics/metrics_service.h	(revision 271296)
+++ chrome/browser/metrics/metrics_service.h	(working copy)
@@ -15,6 +15,7 @@
 #include "base/basictypes.h"
 #include "base/gtest_prod_util.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/memory/scoped_vector.h"
 #include "base/memory/weak_ptr.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/user_metrics.h"
@@ -27,6 +28,7 @@
 #include "chrome/browser/metrics/tracking_synchronizer_observer.h"
 #include "chrome/common/metrics/metrics_service_base.h"
 #include "chrome/installer/util/google_update_settings.h"
+#include "components/metrics/metrics_data_provider.h"

[Ilya Sherman, 2014/05/19 14:45:00]

nit: Can this be forward-declared?

[Alexei Svitkine (slow), 2014/05/19 15:23:02]

Doesn't look like it:

../../base/stl_util.h:44:5: error: deleting pointer to incomplete type
'metrics::MetricsDataProvider' may cause undefined behavior
[-Werror,-Wdelete-incomplete]
    delete *temp;
    ^      ~~~~~
../../base/stl_util.h:140:3: note: in instantiation of function template
specialization
'STLDeleteContainerPointers<__gnu_cxx::__normal_iterator<metrics::MetricsDataProvider
**, std::vector<metrics::MetricsDataProvider *,
std::allocator<metrics::MetricsDataProvider *> > > >' requested here
  STLDeleteContainerPointers(container->begin(), container->end());
  ^
../../base/memory/scoped_vector.h:97:18: note: in instantiation of function
template specialization
'STLDeleteElements<std::vector<metrics::MetricsDataProvider *,
std::allocator<metrics::MetricsDataProvider *> > >' requested here
  void clear() { STLDeleteElements(&v_); }
                 ^
../../base/memory/scoped_vector.h:37:21: note: in instantiation of member
function 'ScopedVector<metrics::MetricsDataProvider>::clear' requested here
  ~ScopedVector() { clear(); }

 #include "content/public/browser/browser_child_process_observer.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
@@ -282,6 +284,10 @@
   // To use this method, SyntheticTrialGroup should friend your class.
   void RegisterSyntheticFieldTrial(const SyntheticTrialGroup& trial_group);
 
+  // Register the specified |provider| to provide additional metrics into the
+  // UMA log. Should be called during MetricsService initialization only.
+  void RegisterDataProvider(scoped_ptr<metrics::MetricsDataProvider> provider);
+
   // Check if this install was cloned or imaged from another machine. If a
   // clone is detected, reset the client id and low entropy source. This
   // should not be called more than once.
@@ -487,6 +493,9 @@
   // low entropy source and whether metrics reporting is enabled. Weak pointer.
   metrics::MetricsStateManager* state_manager_;
 
+  // Additional metrics data providers that have been registered.

[Ilya Sherman, 2014/05/19 14:45:00]

nit: Additional over what?  I'd probably just omit the word, and write this as
"Registered metrics data providers."

[Alexei Svitkine (slow), 2014/05/19 15:23:02]

Done.

+  ScopedVector<metrics::MetricsDataProvider> data_providers_;
+
   base::ActionCallback action_callback_;
 
   content::NotificationRegistrar registrar_;