Add request throttler to thumbnail fetching for articles on mobile NTP

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 b58432248bb663f63c5decdeca1fa6413b0347d1..2adc4ee9957dbb24de1c600bb2dc60a32868303f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -35359,13 +35359,27 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 <histogram name="NewTabPage.RequestThrottler.PerDay" units="requests">
   <owner>jkrcal@chromium.org</owner>
   <summary>
-    Records how many requests of the given type the browser tried to perform
-    each day where the type is specified by the _suffix of the histogram. The
-    histogram is emitted only after midnight of the given day passes - right
-    before the first following request (which can be several days later if the
-    user does not use Chrome in the meantime). The histogram counts requests
-    with both QUOTA_GRANTED and QUOTA_EXCEEDED status, i.e. the count can easily
-    exceed the daily quota. Forced requests are not counted here.
+    Records how many background requests of the given type the browser tried to
+    perform each day where the type is specified by the _suffix of the
+    histogram. The histogram is emitted only after midnight of the given day
+    passes - right before the first following request (which can be several days
+    later if the user does not use Chrome in the meantime). The histogram counts
+    requests with both QUOTA_GRANTED and QUOTA_EXCEEDED status, i.e. the count
+    can easily exceed the daily quota.
+  </summary>
+</histogram>
+
+<histogram name="NewTabPage.RequestThrottler.PerDayInteractive"

[rkaplow, 2016/08/11 17:13:15]

where is this being recorded? I don't see "PerDayInteractive"

[jkrcal, 2016/08/12 09:08:19]

Oh, I was not very focused, yesterday :| Thanks for pointing out! Done.

+    units="requests">
+  <owner>jkrcal@chromium.org</owner>
+  <summary>
+    Records how many interactive requests of the given type the browser tried to
+    perform each day where the type is specified by the _suffix of the
+    histogram. The histogram is emitted only after midnight of the given day
+    passes - right before the first following request (which can be several days
+    later if the user does not use Chrome in the meantime). The histogram counts
+    requests with both QUOTA_GRANTED and QUOTA_EXCEEDED status, i.e. the count
+    can easily exceed the daily quota.
   </summary>
 </histogram>
 
@@ -86288,9 +86302,10 @@ To add a new entry, add it with any value and run test to compute valid value.
 </enum>
 
 <enum name="NtpRequestThrottlerStatus" type="int">

[rkaplow, 2016/08/11 17:13:15]

hm, if you're changing the meaning of the histogram enum, you should replace the
histogram with a new name (ex. suffix with 2). Otherwise the data will get mixed
up

[jkrcal, 2016/08/12 09:08:19]

It is not very obvious but the meaning of the buckets stays the same:
 - terminology changed from "forced" to "interactive",
 - the meaning of the first bucket always was "granted", and 
 - the other two buckets always were meant for "background requests".

-  <int value="0" label="Forced request"/>
-  <int value="1" label="Quota granted"/>
-  <int value="2" label="Quota exceeded"/>
+  <int value="0" label="Interactive request - quota granted"/>
+  <int value="1" label="Background request - quota granted"/>
+  <int value="2" label="Background request - quota exceeded"/>
+  <int value="3" label="Interactive request - quota exceeded"/>
 </enum>
 
 <enum name="NtpSnippetsFetchResult" type="int">
@@ -101635,7 +101650,10 @@ To add a new entry, add it with any value and run test to compute valid value.
 <histogram_suffixes name="RequestThrottlerTypes">
   <suffix name="SuggestionFetcher"
       label="Fetcher for content suggestions on mobile NTP"/>
+  <suffix name="SuggestionThumbnailFetcher"
+      label="Fetcher for article thumbnails on mobile NTP"/>
   <affected-histogram name="NewTabPage.RequestThrottler.PerDay"/>
+  <affected-histogram name="NewTabPage.RequestThrottler.PerDayInteractive"/>
   <affected-histogram name="NewTabPage.RequestThrottler.RequestStatus"/>
 </histogram_suffixes>