Add support for single sample metrics.

base/metrics/single_value_histograms.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_VALUE_HISTOGRAMS_H_
+#define BASE_METRICS_SINGLE_VALUE_HISTOGRAMS_H_
+
+#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 SingleValueCountsHistogram {

[Alexei Svitkine (slow), 2017/04/26 18:12:53]

The convention we use now in base/metrics/histogram_macros.h is for counts
histograms to actually specify their max in the name. So instead of
UMA_HISTOGRAM_COUNTS() we prefer UMA_HISTOGRAM_COUNTS_1M().

However, this interface doesn't seem to be specific to a given type of histogram
- e.g. the same interface can be used for different counts or even sparse/enum
histograms. So I suggest removing Counts from the name.

However, one point of confusion is calling this object a Histogram - which is
already used for other classes that have a different purpose. Maybe
HistogramLogger would be a better name? e.g. SingleValueHistogramLogger?

The comments should also make it clear that it's a single value per instance of
this object, and not ever - e.g. you can still use this to log things many
times, via different instances.

[DaleCurtis, 2017/04/29 00:30:35]

Done.

+ public:
+  virtual ~SingleValueCountsHistogram() {}
+
+  virtual void SetSample(HistogramBase::Sample sample) = 0;
+};
+
+// Factory for creating single value histograms. A single value histogram 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.
+class BASE_EXPORT SingleValueHistogramsFactory {
+ public:
+  virtual ~SingleValueHistogramsFactory() {}
+
+  static SingleValueHistogramsFactory* Get();
+  static void SetFactory(SingleValueHistogramsFactory* factory);
+
+  // Returns a single value histogram for a counts type histogram. Safe to call
+  // from any thread, the returned class must only be used and destroyed from
+  // the same thread as construction.
+  //
+  // See base/metrics/histograms.h for parameter definitions.
+  virtual std::unique_ptr<SingleValueCountsHistogram>
+  CreateSingleValueCountsHistogram(const std::string& name,
+                                   HistogramBase::Sample min,
+                                   HistogramBase::Sample max,
+                                   uint32_t bucket_count) = 0;
+};
+
+}  // namespace base
+
+#endif  // BASE_METRICS_SINGLE_VALUE_HISTOGRAMS_H_