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 8a133ee1a9f3079e510354276b699b4701a5ce6e..6eb5a1826d2f3b7ee8ee9b8249b7d11589d72988 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -53228,6 +53228,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ResourcePrefetchPredictor.DatabaseReadiness" units="%">
+  <owner>alexilin@chromium.org</owner>
+  <summary>
+    An age metric is introduced to show that the predictor database is large
+    enough. Defined as percentage of navigated hosts found in the predictor DB
+    for last X top entries in history, sampled with some probability on each
+    navigation to emit. This metric is only gathered when a minimum number of
+    visits is reached.

[Ilya Sherman, 2017/01/12 21:10:39]

Hmm, I find the wording of this description a bit hard to follow.  Here's a
suggested rewording -- hopefully I understood correctly and kept the semantics
the same!  Please feel free to refine further:

"This metric measures the coverage of the predictor database: On each
navigation, it records what percentage of the top X history entries are found in
the predictor DB.  Caveats: (1) This metric is only recorded once the user has
accumulated enough browsing history, implemented as a minimum threshold of total
page visits by the user.  (2) This metric is sampled, so it's actually only
recorded for 1 out of every K navigations."

If you omit the sampling as I suggest, then I'd drop the "Caveats: (1)" text and
the second caveat.

[alexilin, 2017/01/12 22:53:13]

Thanks Ilya! I can't say better than you suggested.
Let's discuss the sampling problem in the other comment.

+  </summary>
+</histogram>
+
 <histogram name="ResourcePrefetchPredictor.DbStringTooLong">
   <obsolete>
     Deprecated October 2016. No longer recorded.