predictors: Add prefetching hit/miss histograms.

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 a78db125086afabb1b26eff8f927b0ab63c1999b..fce4bc7ce65a4518380ed0f455e877c4d300fc75 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -55122,6 +55122,32 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ResourcePrefetchPredictor.PrefetchHitsCount.Cached">

[alexilin, 2017/02/13 17:41:49]

tiny nit:
specify units for count histograms also e.g
units="count" or units="urls"

[Benoit L, 2017/02/14 08:40:16]

Done.

+  <owner>lizeb@chromium.org</owner>
+  <summary>
+    Number of resources that were prefetched and requested by a page load, for
+    cached requests. Logged after the prefetcher completes.
+  </summary>
+</histogram>
+
+<histogram name="ResourcePrefetchPredictor.PrefetchHitsCount.NotCached">
+  <owner>lizeb@chromium.org</owner>
+  <summary>
+    Number of resources that were prefetched and requested by a page load, for
+    non cached requests. Logged after the prefetcher completes.
+  </summary>
+</histogram>
+
+<histogram name="ResourcePrefetchPredictor.PrefetchHitsSizeKB" units="KB">
+  <owner>lizeb@chromium.org</owner>
+  <summary>
+    Amount of useful data fetched from the network for a prefetch request. This
+    is the total size of the resources accounted for in
+    ResourcePrefetchPredictor.PrefetchHitsCount.NotCached. Logged after the
+    prefetcher completes.
+  </summary>
+</histogram>
+
 <histogram name="ResourcePrefetchPredictor.PrefetchingDuration" units="ms">
   <owner>alexilin@chromium.org</owner>
   <summary>
@@ -55131,6 +55157,33 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ResourcePrefetchPredictor.PrefetchMissesCount.Cached">
+  <owner>lizeb@chromium.org</owner>
+  <summary>
+    Number of resources that were prefetched but not requested by a page load,
+    for cached requests. Logged after the prefetcher completes.
+  </summary>
+</histogram>
+
+<histogram name="ResourcePrefetchPredictor.PrefetchMissesCount.NotCached">
+  <owner>lizeb@chromium.org</owner>
+  <summary>
+    Number of resources that were prefetched but not requested by a page load,
+    for non cached requests, that is waste. Logged after the prefetcher
+    completes.
+  </summary>
+</histogram>
+
+<histogram name="ResourcePrefetchPredictor.PrefetchMissesSizeKB" units="KB">
+  <owner>lizeb@chromium.org</owner>
+  <summary>
+    Amount of wasted data fetched from the network for a prefetch request. This
+    is the total size of the resources accounted for in
+    ResourcePrefetchPredictor.PrefetchMissesCount.NotCached. Logged after the
+    prefetcher completes.
+  </summary>
+</histogram>
+
 <histogram name="ResourcePrefetchPredictor.ReportingEvent"
     enum="ResourcePrefetchPredictorReportingEvent">
   <owner>alexilin@chromium.org</owner>