Add support for single sample metrics.

base/metrics/single_sample_metrics.h

Index: base/metrics/single_sample_metrics.h
diff --git a/base/metrics/single_sample_metrics.h b/base/metrics/single_sample_metrics.h
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+++ b/base/metrics/single_sample_metrics.h
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+// 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.
+//
+// See base/metrics/histogram_macros.h for 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(std::unique_ptr<SingleSampleMetricsFactory> factory);
+
+  // The factory normally persists until process shutdown, but in testing we
+  // should avoid leaking it since it sets a global.
+  static void DeleteFactoryForTesting();
+
+  // The methods below return a single sample metric for counts histograms; see
+  // method comments for the corresponding histogram macro.
+
+  // UMA_HISTOGRAM_CUSTOM_COUNTS()
+  virtual std::unique_ptr<SingleSampleMetric> CreateCustomCountsMetric(
+      const std::string& histogram_name,
+      HistogramBase::Sample min,
+      HistogramBase::Sample max,
+      uint32_t bucket_count) = 0;
+};
+
+// Default implementation for when no factory has been provided to the process.
+// Samples are only recorded within the current process in this case, so samples
+// will be lost in the event of sudden process termination.
+class BASE_EXPORT DefaultSingleSampleMetricsFactory
+    : public SingleSampleMetricsFactory {
+ public:
+  DefaultSingleSampleMetricsFactory() {}
+  ~DefaultSingleSampleMetricsFactory() override {}
+
+  // SingleSampleMetricsFactory:
+  std::unique_ptr<SingleSampleMetric> CreateCustomCountsMetric(
+      const std::string& histogram_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& histogram_name,
+                            HistogramBase::Sample min,
+                            HistogramBase::Sample max,
+                            uint32_t bucket_count,
+                            int32_t flags);
+  ~DefaultSingleSampleMetric() override;
+
+  // SingleSampleMetric:
+  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_