Add new metrics descriptions.

tools/metrics/histograms/histograms.xml

 <!--
 Copyright 2013 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <!--
 This file is used to generate a comprehensive list of Chrome histograms along
 with a detailed description for each histogram.
 
@@ skipping 9175 matching lines @@
   </summary>
 </histogram>
 
 <histogram name="Platform.CrOSEvent" enum="CrosEventEnum">
   <summary>
     Generic event of interest from Chrome OS.  Intended mainly to help assess
     the frequency of rare error conditions.
   </summary>
 </histogram>
 
+<histogram name="Platform.DiskUsage.1User" units="KB">
+  <summary>
+    Statistics on pass-through directory usage. Device has exactly 1 user.
+    Logged once a day, if free disk space is below 1GB.
+  </summary>
+</histogram>
+
+<histogram name="Platform.DiskUsage.2Users" units="KB">
+  <summary>
+    Statistics on pass-through directory usage. Device has exactly 2 users.
+    Logged once a day, if free disk space is below 1GB.
+  </summary>
+</histogram>
+
+<histogram name="Platform.DiskUsage.3Users" units="KB">
+  <summary>
+    Statistics on pass-through directory usage. Device has exactly 3 users.
+    Logged once a day, if free disk space is below 1GB.
+  </summary>
+</histogram>
+
+<histogram name="Platform.DiskUsage.4Users" units="KB">
+  <summary>
+    Statistics on pass-through directory usage. Device has exactly 4 users.
+    Logged once a day, if free disk space is below 1GB.
+  </summary>
+</histogram>
+
+<histogram name="Platform.DiskUsage.5Users" units="KB">
+  <summary>
+    Statistics on pass-through directory usage. Device has exactly 5 users.
+    Logged once a day, if free disk space is below 1GB.
+  </summary>
+</histogram>
+
+<histogram name="Platform.DiskUsage.6Users" units="KB">
+  <summary>
+    Statistics on pass-through directory usage. Device has exactly 6 users.
+    Logged once a day, if free disk space is below 1GB.
+  </summary>
+</histogram>
+
+<histogram name="Platform.DiskUsage.7OrMoreUsers" units="KB">
+  <summary>
+    Statistics on pass-through directory usage. Device has at least 7 users.
+    Logged once a day, if free disk space is below 1GB.
+  </summary>
+</histogram>
+
+<histogram name="Platform.DiskUsage.OldestUserOnDevice">
+  <summary>
+    Days since last login of the least recently user on device. Logged once a
+    day, if disk usage is high.
+  </summary>
+</histogram>
+
+<histogram name="Platform.DiskUsage.UsersOnDevice">
+  <summary>
+    Number of user home dirs on device. Logged once a day, if disk usage is
+    high.
+  </summary>
+</histogram>
+
 <histogram name="Platform.DiskUsageCache" units="KB">
   <summary>KB in use in the /cache filesystem tree. Logged once a day.</summary>
 </histogram>
 
 <histogram name="Platform.DiskUsageChronos" units="KB">
   <summary>
     Chrome OS KB in use in the /home/chronos filesystem tree. Logged once a day
     during log file cleanup.
   </summary>
 </histogram>
@@ skipping 12442 matching lines @@
   <affected-histogram name="Profile.AppCount"/>
 </fieldtrial>
 
 <fieldtrial name="DefaultPinnedApps">
   <group name="Existing"/>
   <group name="Control"/>
   <group name="Alternate"/>
   <affected-histogram name="Cros.ClickOnShelf"/>
 </fieldtrial>
 
+<fieldtrial name="DiskUsagePerDirectory" separator=".">
+  <group name="Cache_Avg"/>

[Alexei Svitkine (slow), 2013/07/24 18:55:17]

Please add a label="..." attribute explaining these.

+  <group name="Cache_Max"/>
+  <group name="Downloads_Avg"/>
+  <group name="Downloads_Max"/>
+  <group name="GCache_Avg"/>
+  <group name="GCache_Max"/>
+  <group name="Bookmark"/>
+  <affected-histogram name="Platform.DiskUsage.1User"/>

[Alexei Svitkine (slow), 2013/07/24 18:55:17]

I'm not sure if you already submitted the other CL, but if you have the
flexibility of changing these on the CrOS CL, it would be better to re-order the
parts of the name, so that 1User, 2Users, 3Users, etc are actual "group" entries
in this model (i.e. suffixes), so that you can have proper descriptions for
Cache vs. Downloads, etc.

Then, your histograms would look like:

Platform.DiskUsage.Cache_Max.2Users

[Denis Kuznetsov (DE-MUC), 2013/07/25 17:04:36]

Done.

+  <affected-histogram name="Platform.DiskUsage.2Users"/>
+  <affected-histogram name="Platform.DiskUsage.3Users"/>
+  <affected-histogram name="Platform.DiskUsage.4Users"/>
+  <affected-histogram name="Platform.DiskUsage.5Users"/>
+  <affected-histogram name="Platform.DiskUsage.6Users"/>
+  <affected-histogram name="Platform.DiskUsage.7OrMoreUsers"/>
+</fieldtrial>
+
 <fieldtrial name="DnsImpact2">
   <group name="disabled_prefetch"
       label="DNS pre-resolving is disabled in these clients"/>
   <group name="disabled_prefetch_4_connections"
       label="DNS pre-resolving is disabled in these clients, and a maximum of
              4 connections per host was allowed"/>
   <group name="enabled_prefetch_4_connections"
       label="a maximum of 4 connections per host was allowed in these clients"/>
   <group name="parallel_4_prefetch"
       label="DNS pre-resolving was only doing 4 concurrent speculative
@@ skipping 1780 matching lines @@
   <group name="Disabled" label="Neither extra webstore link is visible"/>
   <group name="FooterLink" label="Link in bottom right of footer"/>
   <group name="PlusIcon" label="Plus icon in apps page"/>
   <affected-histogram name="Extensions.AppLaunch"/>
   <affected-histogram name="NewTabPage.DefaultPageType"/>
 </fieldtrial>
 
 </fieldtrials>
 
 </histogram-configuration>