Add UMA histograms for data usage tab model

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 ba91367154a04d452aaf5bc017e42717cb163e55..a2574f89e77774b2dfb469619ae38360c5e7e63c 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -6411,6 +6411,73 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 

[tbansal1, 2015/11/16 21:09:13]

For all these UMA descriptions, it would be nice to specify when the sample is
taken (e.g., a sample is taken when a tab entry expires).

[Raj, 2015/11/17 07:04:03]

Done.

+<histogram name="DataUse.TabModel.ExpiredActiveTabEntryRemovalDuration"
+    units="seconds">
+  <owner>bengr@chromium.org</owner>
+  <owner>rajendrant@chromium.org</owner>
+  <summary>
+    The duration in seconds an expired tab entry still actively tracking data
+    usage for a Chrome tab is removed from tab model since its latest data usage
+    tracking session.
+
+    Tab model maintains the tab entries each pertaining to tracking sessions of
+    a single Chrome tab. These entries are considered expired after some
+    duration and can be removed from tab model after that.
+  </summary>
+</histogram>
+
+<histogram name="DataUse.TabModel.ExpiredInactiveTabEntryRemovalDuration"
+    units="seconds">
+  <owner>bengr@chromium.org</owner>
+  <owner>rajendrant@chromium.org</owner>
+  <summary>
+    The duration in seconds an expired tab entry not actively tracking data
+    usage for a Chrome tab is removed from tab model since its latest data usage
+    tracking session.
+
+    Tab model maintains the tab entries each pertaining to tracking sessions of
+    a single Chrome tab. These entries are considered expired after some
+    duration and can be removed from tab model after that.
+  </summary>
+</histogram>
+
+<histogram name="DataUse.TabModel.OldInactiveSessionRemovalDuration"
+    units="seconds">
+  <owner>bengr@chromium.org</owner>
+  <owner>rajendrant@chromium.org</owner>
+  <summary>
+    The amount of time in seconds between the time of deletion of an old
+    inactive data usage tracking session and the time its tracking ended.
+
+    Tab model maintains a limited number of tracking sessions per Chrome tab.
+    When this limit is reached oldest inactive tracking sessions are removed.
+  </summary>
+</histogram>
+
+<histogram name="DataUse.TabModel.TrackingSessionLifetime" units="seconds">
+  <owner>bengr@chromium.org</owner>
+  <owner>rajendrant@chromium.org</owner>
+  <summary>
+    The lifetime of a data usage tracking session of a Chrome tab in seconds,
+    maintained by tab model. This is the duration between the time tracking
+    started due to an entry event and ended with an exit event.
+  </summary>
+</histogram>
+
+<histogram name="DataUse.TabModel.UnexpiredTabEntryRemovalDuration"
+    units="seconds">
+  <owner>bengr@chromium.org</owner>
+  <owner>rajendrant@chromium.org</owner>
+  <summary>
+    The duration in seconds an unexpired tab entry is removed from tab model and
+    its latest data usage tracking session.
+
+    Tab model maintains a limited number of entries each pertaining to tracking
+    sessions of a single Chrome tab. When this limit is reached even after
+    removing expired tab entries, then oldest unexpired tab entries are removed.
+  </summary>
+</histogram>
+
 <histogram name="DataUse.TrafficSize.System" units="bytes">
   <owner>amohammadkhan@chromium.org</owner>
   <owner>bengr@chromium.org</owner>