Introduce a MetricsDataProvider interface.

components/metrics.gypi

 # 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.
 
 {
   'targets': [
     {
       'target_name': 'metrics',
       'type': 'static_library',
       'include_dirs': [
         '..',
       ],
       'dependencies': [
         '../base/base.gyp:base',
         'component_metrics_proto',
       ],
       'sources': [
+        'metrics/metrics_provider.h',
         'metrics/metrics_hashes.cc',
         'metrics/metrics_hashes.h',
         'metrics/metrics_log_base.cc',
         'metrics/metrics_log_base.h',
         'metrics/metrics_log_manager.cc',
         'metrics/metrics_log_manager.h',
         'metrics/metrics_pref_names.cc',
         'metrics/metrics_pref_names.h',
         'metrics/metrics_service_client.h',
         'metrics/metrics_service_observer.cc',
@@ skipping 43 matching lines @@
             'metrics/chromeos/metric_sample.h',
           ],
           'dependencies': [
             '../base/base.gyp:base',
           ],
         },
       ],
     }],
   ],
 }