Add a histogram for recording content suggestions local usage time stats.

components/ntp_snippets/content_suggestions_metrics.cc

Index: components/ntp_snippets/content_suggestions_metrics.cc
diff --git a/components/ntp_snippets/content_suggestions_metrics.cc b/components/ntp_snippets/content_suggestions_metrics.cc
index 4117fae096bb855d15ecd5949833e97b1e046919..b4f2417ebc8370459f4566d73bf94120c9680ef7 100644
--- a/components/ntp_snippets/content_suggestions_metrics.cc
+++ b/components/ntp_snippets/content_suggestions_metrics.cc
@@ -40,6 +40,8 @@ const char kHistogramDismissedUnvisited[] =
     "NewTabPage.ContentSuggestions.DismissedUnvisited";
 const char kHistogramDismissedVisited[] =
     "NewTabPage.ContentSuggestions.DismissedVisited";
+const char kHistogramUsageTimeLocal[] =
+    "NewTabPage.ContentSuggestions.UsageTimeLocal";
 const char kHistogramVisitDuration[] =
     "NewTabPage.ContentSuggestions.VisitDuration";
 const char kHistogramMoreButtonShown[] =
@@ -159,6 +161,24 @@ void LogCategoryHistogramScore(const char* base_name,
   UmaHistogramScore(name, score);
 }
 
+// Records ContentSuggestions usage. Therefore the day is sliced into 20min
+// buckets. Depending on the current local time the count of the corresponding
+// bucket is increased.

[Marc Treib, 2016/11/16 14:14:29]

Mention that "usage" means "impression" rather than "click"?

[markusheintz_, 2016/11/16 14:52:13]

It will be more that just the impression, i'll also count how often an article
was opened and other usages. I'll add counting article open events.

[Marc Treib, 2016/11/16 15:10:08]

Then all the more important to define what we consider a "usage" :)

Is impression + click all that will be tracked in this histogram? If not, it'd
probably be best to add everything else in this CL too - it's considered bad
practice to change the definition of a histogram after it's been introduced.

+void RecordContentSuggestionsUsage() {
+  const int kBucketSize = 20;  // min

[Marc Treib, 2016/11/16 14:14:29]

nit: kBucketSizeMins and remove the comment?

[markusheintz_, 2016/11/16 14:52:13]

Done.

+  const int kNumBuckets = 24 * 60 / kBucketSize;
+
+  base::Time now(base::Time::Now());
+  base::Time::Exploded now_exploded;
+  now.LocalExplode(&now_exploded);

[Marc Treib, 2016/11/16 14:14:29]

nit: You could just write
base::Time::Now().LocalExplode(...)

[markusheintz_, 2016/11/16 14:52:13]

Done.

+  size_t bucket = (now_exploded.hour * 60 + now_exploded.minute) / kBucketSize;

[Marc Treib, 2016/11/16 14:14:29]

Why size_t rather than int?

[markusheintz_, 2016/11/16 14:52:13]

bucket size can't be negative.

[Marc Treib, 2016/11/16 15:10:08]

Nothing here can be negative, yet everything else is an int. :)
Per the style guide, we don't use unsigned types simply because the value
shouldn't be negative:
https://google.github.io/styleguide/cppguide.html#Integer_Types

+
+  UMA_HISTOGRAM_ENUMERATION(
+       kHistogramUsageTimeLocal,
+       bucket,
+       kNumBuckets);

[Marc Treib, 2016/11/16 14:14:29]

nit: run "git cl format"

[markusheintz_, 2016/11/16 14:52:13]

Done.

+}
+
 }  // namespace
 
 void OnPageShown(
@@ -188,6 +208,13 @@ void OnSuggestionShown(int global_position,
   LogCategoryHistogramAge(kHistogramShownAge, category, age);
 
   LogCategoryHistogramScore(kHistogramShownScore, category, score);
+
+  // When the first suggestion of all snippets is shown, then we count this as a

[Marc Treib, 2016/11/16 14:14:29]

"When the first of all suggestions is shown, ..."? Let's try not to use
"snippet" anymore.

[markusheintz_, 2016/11/16 14:52:13]

Done.

+  // single usage of content suggestions.
+  // TODO(markusheintz): Use position 0 once http://crbug.com/664539 is fixed.

[Marc Treib, 2016/11/16 14:14:29]

It'll be fixed very soon, see https://codereview.chromium.org/2504963002, so
please use 0 already :)

[markusheintz_, 2016/11/16 14:52:13]

Done.

+  if (global_position == 1) {
+    RecordContentSuggestionsUsage();
+  }
 }
 
 void OnSuggestionOpened(int global_position,