Reland "Add a HistogramRecorder class and use cases."

base/test/histogram_recorder.h

+// Copyright 2013 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 BASE_TEST_HISTOGRAM_RECORDER_H_
+#define BASE_TEST_HISTOGRAM_RECORDER_H_
+
+#include <map>
+
+#include "base/memory/scoped_ptr.h"
+#include "base/metrics/histogram_samples.h"
+
+namespace base {
+
+// This class acts as a differential reader for histogram samples, enabling
+// tests to check that metrics were sent as they should be.

[ppi, 2013/12/19 16:57:27]

Should we say "recorded" instead of "sent"? Am I correct that they are always
recorded, but not always sent?

[lpromero, 2013/12/20 11:53:44]

Yes, you are correct. By sent, I meant triggered from the code to the histogram
"engine", not sent from the histogram engine to our servers. I agree that
"record" is a better wording. Changing here and everywhere.

+class HistogramRecorder {
+ public:
+  // Initializes the HistogramRecorder system.
+  static void Initialize();
+  HistogramRecorder();
+  virtual ~HistogramRecorder();
+
+  // Returns whether the HistogramRecorder has been initialized.
+  static bool IsActive();
+
+  // Returns the histogram data accumulated since this instance was created.
+  // Returns NULL if no samples are available.
+  scoped_ptr<HistogramSamples> GetHistogramSamplesSinceCreation(
+      const std::string& histogram_name);
+
+ private:
+  // Used to determine the histogram changes made during this instance's
+  // lifecycle.

[ppi, 2013/12/19 16:57:27]

Could we clarify the ownership of HistogramSamples stored in the map in a
comment? Raw pointers seem to indicate that someone else is owning and deleting
them.

[lpromero, 2013/12/20 11:53:44]

Done.

+  std::map<std::string, HistogramSamples*> original_samples_;

[ppi, 2013/12/19 16:57:27]

DISALLOW_COPY_AND_ASSIGN(HistogramRecorder);

[lpromero, 2013/12/20 11:53:44]

Done.

+};
+
+}  // namespace base
+
+#endif  // BASE_TEST_HISTOGRAM_RECORDER_H_