Record detailed breakdown of Reader Mode triggering

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 9087c187d2f21e8c7d6daa12ec102e82489f20dc..4577717166333b78442e935b10bb59907dd50b80 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -9612,6 +9612,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="DomDistiller.DistillabilityRejection" enum="DistillRejection">
+  <owner>wychen@chromium.org</owner>
+  <summary>
+    The reason to reject distillability at PageDistillableAfterParsing time.
+  </summary>
+</histogram>
+
 <histogram name="DomDistiller.DistillationQuality" enum="BooleanSuccess">
   <owner>nyquist@chromium.org</owner>
   <summary>
@@ -66775,6 +66782,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="3" label="Mobile-friendly, distillable"/>
 </enum>
 
+<enum name="DistillRejection" type="int">
+  <int value="0" label="Not an article"/>
+  <int value="1" label="Mobile-friendly"/>
+  <int value="2" label="Domain is blacklisted"/>
+  <int value="3" label="Predicted to be short"/>
+  <int value="4" label="Not rejected"/>
+</enum>
+
 <enum name="DllHash" type="int">
 <!-- Generated by chrome_elf/dll_hash/dll_hash_main.cc -->