Move more metrics classes to metrics namespace.

components/metrics/metrics_service_observer.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_SERVICE_OBSERVER_H_
 #define COMPONENTS_METRICS_METRICS_SERVICE_OBSERVER_H_
 
 #include "base/macros.h"
 
+namespace metrics {
+
 // MetricsServiceObserver receives notifications from MetricsService.
 // An observer must be added, removed, and notified on the same thread.
 class MetricsServiceObserver {
  public:
   // Called when a new MetricsLog is created.
   virtual void OnDidCreateMetricsLog() = 0;
 
  protected:
   MetricsServiceObserver();
   virtual ~MetricsServiceObserver();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MetricsServiceObserver);
 };
 
+}  // namespace metrics
+
 #endif  // COMPONENTS_METRICS_METRICS_SERVICE_OBSERVER_H_