Add support for single sample metrics.

components/metrics/single_sample_metrics_factory_impl.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 COMPONENTS_METRICS_SINGLE_VALUE_HISTOGRAM_FACTORY_IMPL_H_
+#define COMPONENTS_METRICS_SINGLE_VALUE_HISTOGRAM_FACTORY_IMPL_H_
+
+#include <string>
+
+#include "base/metrics/single_sample_metrics.h"
+#include "base/threading/thread_local.h"
+#include "components/metrics/public/interfaces/single_sample_metrics.mojom.h"
+
+namespace metrics {
+
+// SingleSampleMetricsFactory implementation for creating SingleSampleMetric
+// instances that communicate over mojo to instances in another process.
+//
+// Persistance outside of the current process allows these metrics to record a
+// sample even in the event of sudden process termination. As an example, this
+// is useful for garbage collected objects which may never get a chance to run
+// their destructors in the event of a fast shutdown event (process kill).
+class SingleSampleMetricsFactoryImpl : public base::SingleSampleMetricsFactory {
+ public:
+  using CreateProviderCB =
+      base::RepeatingCallback<void(mojom::SingleSampleMetricsProviderRequest)>;
+
+  // Constructs a factory capable of vending single sample metrics from any
+  // thread. |create_provider_cb| will be called from arbitrary threads to
+  // create providers as necessary; the callback must handle thread safety.
+  explicit SingleSampleMetricsFactoryImpl(CreateProviderCB create_provider_cb);

[Alexei Svitkine (slow), 2017/05/03 21:18:48]

Can you mention somewhere in the comments the purpose of the callback?

[DaleCurtis, 2017/05/03 21:37:52]

What's do you think is missing from the current "|create_provider_cb| will be
called..." sentence?

[Alexei Svitkine (slow), 2017/05/03 21:45:08]

I meant to explain why it's necessary for it be provided externally - e.g.
what's the use case for passing different callbacks to this?

[DaleCurtis, 2017/05/04 02:28:59]

I'm not sure what you mean? Probably clients will always pass the same callback
for construction.

As for why a callback? We pass a callback here so this code doesn't have to take
DEPS on content for service names or require a service_manager::Connector() to
construct for testing (which is a huge pain since that's a whole custom test
harness).

[Alexei Svitkine (slow), 2017/05/04 14:15:08]

Sorry for the confusion - I meant that your explanation above (about DEPS /
service names) could be added to the comment. That part explains why we're using
a callback here.

[DaleCurtis, 2017/05/04 18:55:27]

Ah I see, done.

+  ~SingleSampleMetricsFactoryImpl() override;
+
+  // base::SingleSampleMetricsFactory:
+  std::unique_ptr<base::SingleSampleMetric> CreateCustomCountsMetric(
+      const std::string& histogram_name,
+      base::HistogramBase::Sample min,
+      base::HistogramBase::Sample max,
+      uint32_t bucket_count) override;
+
+  // Providers live forever in production, but tests should be kind and clean up
+  // after themselves to avoid tests trampling on one another. Destroys the
+  // provider in the TLS slot for the calling thread.
+  void DestroyProviderForTesting();
+
+ private:
+  // Creates a single sample metric.
+  std::unique_ptr<base::SingleSampleMetric> CreateMetric(
+      const std::string& histogram_name,
+      base::HistogramBase::Sample min,
+      base::HistogramBase::Sample max,
+      uint32_t bucket_count,
+      int32_t flags);
+
+  // Gets the SingleSampleMetricsProvider for the current thread. If none
+  // exists, then a new instance is created and set in the TLS slot.
+  mojom::SingleSampleMetricsProvider* GetProvider();
+
+  CreateProviderCB create_provider_cb_;
+
+  // Per thread storage slot for the mojo provider.
+  base::ThreadLocalPointer<mojom::SingleSampleMetricsProviderPtr> provider_;
+
+  DISALLOW_COPY_AND_ASSIGN(SingleSampleMetricsFactoryImpl);
+};
+
+}  // namespace metrics
+
+#endif  // COMPONENTS_METRICS_SINGLE_VALUE_HISTOGRAM_FACTORY_IMPL_H_