Change the logic for discarding all non-histogram logs because of actions/omnibox events to truncat…

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 6ed192b3a430a9c42cd098e58111a7082c27aa59..e60bf3d34496f7e47a63e7661eaa434ae0728785 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -77719,6 +77719,9 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="UMA.Discarded Log Events">
+  <obsolete>
+    Replaced by UMA.Truncate* metrics.
+  </obsolete>
   <owner>asvitkine@chromium.org</owner>
   <owner>rkaplow@chromium.org</owner>
   <summary>
@@ -78209,6 +78212,22 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="UMA.TruncateOmnibox" units="events">

[Alexei Svitkine (slow), 2017/05/30 21:57:00]

Nit: Make the metric name more clearly reflect what this is.

e.g. UMA.TruncatedEvents.Omnibox

The above syntax also allows to use histogram_suffixes - although for just 2
it's probably fine without it, but if we expand this to more events in the
future, we can still keep the same naming conventions.

[rkaplow, 2017/05/31 15:17:06]

Done.

+  <owner>rkaplow@chromium.org</owner>
+  <summary>
+    The number of omnibox events that existed in the metrics log before we
+    truncated them to save bandwidth.
+  </summary>
+</histogram>
+
+<histogram name="UMA.TruncateUserAction" units="events">
+  <owner>rkaplow@chromium.org</owner>
+  <summary>
+    The number of user actions events that existed in the metrics log before we
+    truncated them to save bandwidth.
+  </summary>
+</histogram>
+
 <histogram name="UMA.Unacceptable_Log_Discarded">
   <obsolete>
     Deprecated as of May, 2012 (i.e. Chrome 21+).  Replaced by the