Introduce a MetricsDataProvider interface.

components/metrics/metrics_data_provider.h

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_METRICS_METRICS_DATA_PROVIDER_H_
+#define COMPONENTS_METRICS_METRICS_DATA_PROVIDER_H_
+
+#include "base/basictypes.h"
+
+namespace metrics {
+
+class ChromeUserMetricsExtension;
+class SystemProfileProto;
+class SystemProfileProto_Stability;
+
+// MetricsDataProvider is an interface allowing different parts of the UMA
+// protos to be filled out by different classes.
+class MetricsDataProvider {
+ public:
+  MetricsDataProvider() {}
+  virtual ~MetricsDataProvider() {}
+
+  // Provides additional metrics into the system profile. Default implementation
+  // is a no-op.

[Ilya Sherman, 2014/05/19 16:05:53]

nit: Probably no need to mention the default implementation for this method,
unless you think it's worth mentioning for all the methods.

[Alexei Svitkine (slow), 2014/05/19 16:08:57]

Done.

+  virtual void ProvideSystemProfileMetrics(
+      SystemProfileProto* system_profile_proto) {}
+
+  // Provides additional stability metrics. Stability metrics can be provided
+  // directly into |stability_proto| fields or by logging stability histograms
+  // via the UMA_STABILITY_HISTOGRAM_ENUMERATION() macro.
+  virtual void ProvideStabilityMetrics(
+      SystemProfileProto_Stability* stability_proto) {}
+
+  // Provides general metrics that are neither system profile nor stability
+  // metrics.
+  virtual void ProvideGeneralMetrics(
+      ChromeUserMetricsExtension* uma_proto) {}
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(MetricsDataProvider);
+};
+
+}  // namespace metrics
+
+#endif  // COMPONENTS_METRICS_METRICS_DATA_PROVIDER_H_