Add support for single sample metrics.

components/metrics/BUILD.gn

Index: components/metrics/BUILD.gn
diff --git a/components/metrics/BUILD.gn b/components/metrics/BUILD.gn
index 3852c12e5dd9d5588dd087ef3e51c2e51fa98373..32330c8ecd5609e3147070a4fddd52f9957a560b 100644
--- a/components/metrics/BUILD.gn
+++ b/components/metrics/BUILD.gn
@@ -91,8 +91,11 @@ static_library("metrics") {
   ]
 
   public_deps = [
+    ":single_sample_metrics",
     "//components/metrics/proto",
+    "//components/metrics/public/interfaces:single_sample_metrics_mojo_bindings",
   ]
+
   deps = [
     ":call_stack_profile_params",
     "//base",
@@ -248,6 +251,20 @@ if (!is_ios) {
   }
 }
 
+source_set("single_sample_metrics") {
+  sources = [
+    "single_sample_metrics_factory_impl.cc",
+    "single_sample_metrics_factory_impl.h",
+    "single_sample_metrics_provider.cc",

[Alexei Svitkine (slow), 2017/05/03 21:18:47]

Hmm, I find this naming unfortunate - because we already have a notion of
"metrics provider" that is something else - and I'd rather not add confusion by
naming this also a "metrics provider".

[DaleCurtis, 2017/05/03 21:37:51]

This is naming convention used by mojo for these type of objects. So we're in
trouble either way it sounds like. I.e. break convention in mojo or break
convention in metrics. I don't have a strong opinion either way, so up to you.
Do you have a naming suggestion?

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

Perhaps we can put something between "metrics" and "provider" so it's not using
those words together, at least?

e.g. "single_sample_metrics_mojo_provider.cc"?

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

Okay, I think I have a compromise which works. I've removed this source file and
deduped the tests by making the implementation of
mojom::SingleSampleMetricsProvider an internal detail of the static helpers. We
still have a mojo class and a private class which end in MetricsProvider, but
they are not exposed elsewhere. WDYT?

[DaleCurtis, 2017/05/04 03:09:24]

Ah, actually this doesn't work since AddInterface() needs to infer the
implementation name from the ::Create method. So we must expose the class
somehow. I'll redo this tomorrow.

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

Oooh, fixed my template errors so this is working now. PTAL.

+    "single_sample_metrics_provider.h",
+  ]
+
+  deps = [
+    "//components/metrics/public/interfaces:single_sample_metrics_mojo_bindings",
+    "//mojo/public/cpp/bindings",
+  ]
+}
+
 source_set("call_stack_profile_params") {
   sources = [
     "call_stack_profile_params.cc",
@@ -335,6 +352,8 @@ source_set("unit_tests") {
     "persisted_logs_unittest.cc",
     "profiler/profiler_metrics_provider_unittest.cc",
     "profiler/tracking_synchronizer_unittest.cc",
+    "single_sample_metrics_factory_impl_unittest.cc",
+    "single_sample_metrics_provider_unittest.cc",
     "stability_metrics_helper_unittest.cc",
     "stability_metrics_provider_unittest.cc",
     "ui/screen_info_metrics_provider_unittest.cc",