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..32d52cfdb7ef64ccc31854d474e68db2e9deb164 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[] =
@@ -49,9 +51,8 @@ const char kHistogramMoreButtonClicked[] =
 
 const char kPerCategoryHistogramFormat[] = "%s.%s";
 
-// Each suffix here should correspond to an entry under histogram suffix
-// ContentSuggestionCategory in histograms.xml.
-std::string GetCategorySuffix(Category category) {
+// Returns the KnownCategory value for the given Category.
+KnownCategories GetKnownCategory(Category category) {

[Marc Treib, 2016/11/16 16:00:55]

Per the comments below, this isn't necessary. (I wouldn't like to pass around a
KnownCategories instance that isn't necessarily one of the actual values...)

[markusheintz_, 2016/11/16 16:09:42]

Done.

   static_assert(
       std::is_same<decltype(category.id()), typename base::underlying_type<
                                                 KnownCategories>::type>::value,
@@ -60,7 +61,13 @@ std::string GetCategorySuffix(Category category) {
   // cast from int to KnownCategories, even if the given value isn't listed in
   // the enumeration. The switch still makes sure that all known values are
   // listed here.
-  KnownCategories known_category = static_cast<KnownCategories>(category.id());
+  return static_cast<KnownCategories>(category.id());
+}
+
+// Each suffix here should correspond to an entry under histogram suffix
+// ContentSuggestionCategory in histograms.xml.
+std::string GetCategorySuffix(Category category) {
+  KnownCategories known_category = GetKnownCategory(category);
   switch (known_category) {
     case KnownCategories::RECENT_TABS:
       return "RecentTabs";
@@ -159,6 +166,21 @@ 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.
+void RecordContentSuggestionsUsage() {
+  const int kBucketSizeMins = 20;
+  const int kNumBuckets = 24 * 60 / kBucketSizeMins;
+
+  base::Time::Exploded now_exploded;
+  base::Time::Now().LocalExplode(&now_exploded);
+  size_t bucket =
+      (now_exploded.hour * 60 + now_exploded.minute) / kBucketSizeMins;
+
+  UMA_HISTOGRAM_ENUMERATION(kHistogramUsageTimeLocal, bucket, kNumBuckets);
+}
+
 }  // namespace
 
 void OnPageShown(
@@ -188,6 +210,13 @@ void OnSuggestionShown(int global_position,
   LogCategoryHistogramAge(kHistogramShownAge, category, age);
 
   LogCategoryHistogramScore(kHistogramShownScore, category, score);
+
+  // When the first of all suggestions is shown, then we count this as a

[Marc Treib, 2016/11/16 16:00:55]

nit: Now it's not the first of all suggestions anymore :)

[markusheintz_, 2016/11/16 16:09:42]

Done.

+  // single usage of content suggestions.
+  if (GetKnownCategory(category) == KnownCategories::ARTICLES &&

[Marc Treib, 2016/11/16 16:00:55]

category.IsKnownCategory(KnownCategories::ARTICLES)

[markusheintz_, 2016/11/16 16:09:42]

Done.

+      category_position == 0) {
+    RecordContentSuggestionsUsage();
+  }
 }
 
 void OnSuggestionOpened(int global_position,
@@ -212,6 +241,10 @@ void OnSuggestionOpened(int global_position,
   LogCategoryHistogramEnumeration(
       kHistogramOpenDisposition, category, static_cast<int>(disposition),
       static_cast<int>(WindowOpenDisposition::MAX_VALUE) + 1);
+
+  if (GetKnownCategory(category) == KnownCategories::ARTICLES) {

[Marc Treib, 2016/11/16 16:00:55]

Also here:
category.IsKnownCategory(KnownCategories::ARTICLES)

[markusheintz_, 2016/11/16 16:09:42]

Done.

+    RecordContentSuggestionsUsage();
+  }
 }
 
 void OnSuggestionMenuOpened(int global_position,