Add support for single sample metrics.

base/metrics/single_sample_metrics.h

+// Copyright 2017 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_METRICS_SINGLE_SAMPLE_METRICS_H_
+#define BASE_METRICS_SINGLE_SAMPLE_METRICS_H_
+
+#include <string>
+
+#include "base/base_export.h"
+#include "base/macros.h"
+#include "base/metrics/histogram_base.h"
+
+namespace base {
+
+// See base/metrics/histograms.h for parameter definitions. Must only be used
+// and destroyed from the same thread as construction.
+class BASE_EXPORT SingleSampleMetric {
+ public:
+  virtual ~SingleSampleMetric() {}
+
+  virtual void SetSample(HistogramBase::Sample sample) = 0;
+};
+
+// Factory for creating single sample metrics. A single sample metric only
+// reports its sample once at destruction time. The sample may be changed prior
+// to destruction using the SetSample() method as many times as desired.
+//
+// The metric creation methods are safe to call from any thread, however the
+// returned class must only be used and destroyed from the same thread as
+// construction.
+//
+// |name| is the name of the histogram. See base/metrics/histogram_macros.h for

[Alexei Svitkine (slow), 2017/05/03 19:59:46]

Nit: I'd actually just name the param |histogram_name| and remove this sentence
from this paragraph.

[DaleCurtis, 2017/05/03 20:54:47]

Done.

+// usage recommendations and base/metrics/histogram.h for full parameter
+// definitions.
+class BASE_EXPORT SingleSampleMetricsFactory {
+ public:
+  virtual ~SingleSampleMetricsFactory() {}
+
+  // Returns the factory provided by SetFactory(), or if no factory has been set
+  // a default factory will be provided (future calls to SetFactory() will fail
+  // if the default factory is ever vended).
+  static SingleSampleMetricsFactory* Get();
+  static void SetFactory(SingleSampleMetricsFactory* factory);
+
+  // The factory normally persist until process shutdown, but in testing we

[Alexei Svitkine (slow), 2017/05/03 19:59:46]

Nit: persist -> persists

[DaleCurtis, 2017/05/03 20:54:47]

Done.

+  // should avoid leaking it since it sets a global.
+  static void DeleteFactoryForTesting();
+
+  // The methods below return a single sample metric for counts histograms.

[Alexei Svitkine (slow), 2017/05/03 19:59:46]

Nit: Mention corresponding macro this is a parralel to.

[DaleCurtis, 2017/05/03 20:54:47]

Done.

+  virtual std::unique_ptr<SingleSampleMetric> CreateCustomCountsMetric(
+      const std::string& name,
+      HistogramBase::Sample min,
+      HistogramBase::Sample max,
+      uint32_t bucket_count) = 0;
+};
+
+// Default implementation for when no factory has been provided to the process.

[Alexei Svitkine (slow), 2017/05/03 19:59:46]

Nit: Expand comment to mention data is logged locally by this instance.

[DaleCurtis, 2017/05/03 20:54:47]

Done.

+class BASE_EXPORT DefaultSingleSampleMetricsFactory
+    : public SingleSampleMetricsFactory {
+ public:
+  DefaultSingleSampleMetricsFactory() {}
+  ~DefaultSingleSampleMetricsFactory() override {}
+
+  // SingleSampleMetricsFactory implementation.

[Alexei Svitkine (slow), 2017/05/03 19:59:46]

Nit: Prefer "// SingleSampleMetricsFactory:" syntax for these comments for new
code. Here and below.

[DaleCurtis, 2017/05/03 20:54:47]

Done.

+  std::unique_ptr<SingleSampleMetric> CreateCustomCountsMetric(
+      const std::string& name,
+      HistogramBase::Sample min,
+      HistogramBase::Sample max,
+      uint32_t bucket_count) override;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(DefaultSingleSampleMetricsFactory);
+};
+
+class BASE_EXPORT DefaultSingleSampleMetric : public SingleSampleMetric {
+ public:
+  DefaultSingleSampleMetric(const std::string& name,

[Alexei Svitkine (slow), 2017/05/03 19:59:46]

Nit: histogram_name

[DaleCurtis, 2017/05/03 20:54:47]

Done.

+                            HistogramBase::Sample min,
+                            HistogramBase::Sample max,
+                            uint32_t bucket_count,
+                            int32_t flags);
+  ~DefaultSingleSampleMetric() override;
+
+  // SingleSampleMetric implementation.
+  void SetSample(HistogramBase::Sample sample) override;
+
+ private:
+  HistogramBase* const histogram_;
+
+  // The last sample provided to SetSample(). We use -1 as a sentinel value to
+  // indicate no sample has been set.
+  HistogramBase::Sample sample_ = -1;
+
+  DISALLOW_COPY_AND_ASSIGN(DefaultSingleSampleMetric);
+};
+
+}  // namespace base
+
+#endif  // BASE_METRICS_SINGLE_SAMPLE_METRICS_H_