tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 5ff826f91882fdafd855d07aba96a376fed17b42..3afdc9ae08bc52198b1c7f323696fc25c534e09b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -39001,6 +39001,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="NewTabPage.ContentSuggestions.OpenedCategoryRank"
+    units="index">
+  <owner>vitaliii@chromium.org</owner>
+  <summary>
+    Android: The rank of a category on the NTP, whose suggestion card is clicked
+    through to the host website of the content. This tracked rank can be
+    different from the position observed by the user, e.g. when some categories
+    are not shown (they are empty or dismissed by the user).

[Marc Treib, 2017/01/16 10:45:42]

I don't understand the part about empty categories. Are they counted or not? I
think if a category shows up (empty or not), then it should be counted.

[vitaliii, 2017/01/18 09:19:20]

Done.

However, they won't be counted after https://codereview.chromium.org/2618893003.

[Marc Treib, 2017/01/18 09:30:04]

...so right now, we'll count empty categories, even if they're not shown to the
user at all?! Under what circumstances exactly? E.g. would we always count
"bookmarks" even if the user doesn't have any?

[vitaliii, 2017/01/18 09:36:30]

Yes.

The idea was to evaluate how ranker performs. 
E.g. imagine there are 10 categories and only 1 of them is shown and it is
ranked last by the ranker. The metric above would show the ranker is doing bad
job, while ignoring empty ones wouldn't.

However, both approaches should become equivalent after some time (the situation
above is rare and if a category is empty, than it won't get any clicks), thus, I
do not mind either of them.

[Marc Treib, 2017/01/18 09:39:13]

Okay, fair enough I guess. Thanks for the explanation!

+  </summary>
+</histogram>
+
 <histogram name="NewTabPage.ContentSuggestions.OpenedScore" units="score">
   <owner>treib@chromium.org</owner>
   <summary>
@@ -110175,6 +110186,7 @@ value.
   <affected-histogram name="NewTabPage.ContentSuggestions.OpenDisposition"/>
   <affected-histogram name="NewTabPage.ContentSuggestions.Opened"/>
   <affected-histogram name="NewTabPage.ContentSuggestions.OpenedAge"/>
+  <affected-histogram name="NewTabPage.ContentSuggestions.OpenedCategoryRank"/>
   <affected-histogram name="NewTabPage.ContentSuggestions.OpenedScore"/>
   <affected-histogram name="NewTabPage.ContentSuggestions.Shown"/>
   <affected-histogram name="NewTabPage.ContentSuggestions.ShownAge"/>