[remote suggestions] Add a new histogram for recording the time since a snippet was fetched.

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 95a1381cc08ef963bcdad3cefe72076adce86337..ba6f2b30bc0b7b59ebafb038d463c134ad9439b0 100644
--- a/components/ntp_snippets/content_suggestions_metrics.cc
+++ b/components/ntp_snippets/content_suggestions_metrics.cc
@@ -61,6 +61,8 @@ const char kHistogramCategoryDismissed[] =
     "NewTabPage.ContentSuggestions.CategoryDismissed";
 const char kHistogramContentSuggestionsTimeSinceLastBackgroundFetch[] =
     "NewTabPage.ContentSuggestions.TimeSinceLastBackgroundFetch";
+const char kHistogramContentSuggestionsTimeSinceSuggestionFetched[] =

[Marc Treib, 2017/02/21 09:14:09]

optional nit: You could remove "ContentSuggestions" from the variable name, also
from the one above. All the other histograms don't have it, and it's obvious
from context anyway.

[markusheintz_, 2017/02/21 11:31:50]

Done.

+    "NewTabPage.ContentSuggestions.TimeSinceSuggestionFetched";
 
 const char kPerCategoryHistogramFormat[] = "%s.%s";
 
@@ -235,6 +237,13 @@ void OnSuggestionShown(int global_position,
 
   LogCategoryHistogramScore(kHistogramShownScore, category, score);
 
+  // Records the time since the fetch time of the displayed snippet.
+  UMA_HISTOGRAM_CUSTOM_TIMES(
+      kHistogramContentSuggestionsTimeSinceSuggestionFetched,
+      base::Time::Now() - fetch_date, base::TimeDelta::FromSeconds(1),
+      base::TimeDelta::FromDays(7),
+      /*bucket_count=*/100);

[Marc Treib, 2017/02/21 09:14:09]

Should we record this only for ARTICLES?
If not, should it be a per-category histogram, and/or should we only record it
if we actually have a fetch date?

[markusheintz_, 2017/02/21 11:31:50]

Done.

No the fetch date for non Article suggestions should always be 0.

+
   // TODO(markusheintz): Discuss whether the code below should be move into a
   // separate method called OnSuggestionsListShown.
   // When the first of the articles suggestions is shown, then we count this as