chromeos: Add descriptions for power manager metrics.

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 22ab0b304a14ee9ed1da3920f74429a981173668..917a4bc36c022657f72ad5f0f7e712a69a2a588f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -6278,13 +6278,15 @@ other types of suffix sets.
 
 <histogram name="Power.BacklightLevelOnAC" units="%">
   <summary>
-    The level of the backlight as a percentage when the user is on AC.
+    The level of the backlight as a percentage when the user is on AC. Sampled
+    periodically.

[Alexei Svitkine (slow), 2013/05/24 14:48:47]

Nit: Mention how frequently. Here and in the other ones.

[Daniel Erat, 2013/05/24 15:00:23]

Sure (although I'm concerned about the likelihood of something like this getting
out-of-date).

   </summary>
 </histogram>
 
 <histogram name="Power.BacklightLevelOnBattery" units="%">
   <summary>
     The level of the backlight as a percentage when the user is on battery.
+    Sampled periodically.
   </summary>
 </histogram>
 
@@ -6295,13 +6297,22 @@ other types of suffix sets.
   </summary>
 </histogram>
 
-<histogram name="Power.BatteryDischargeRate">
+<histogram name="Power.BatteryDischargeRate" units="mW">
   <summary>
     Chrome OS battery discharge rate in mW sampled when the device runs on
     battery.
   </summary>
 </histogram>
 
+<histogram name="Power.BatteryDischargeRateWhileSuspended" units="mW">
+  <summary>
+    Chrome OS battery discharge rate in mW while the system was suspended,
+    sampled at resume.  Only reported if the system was on battery power both
+    before suspending and after resuming and if the energy level didn't increase
+    while suspended (which would indicate that an AC adapter was connected).
+  </summary>
+</histogram>
+
 <histogram name="Power.BatteryInfoSample" enum="BatteryInfoSampleResult">
   <summary>
     Counts the number of times we have read the battery status from sysfs and if
@@ -6480,6 +6491,19 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Power.KeyboardBacklightLevel" units="%">
+  <summary>
+    The level of the keyboard backlight as a percentage. Sampled periodically.
+  </summary>
+</histogram>
+
+<histogram name="Power.KeyboardBrightnessAdjust" enum="PowerBrightnessAdjust">
+  <summary>
+    Number of times the user has adjusted keyboard backlight brightness up and
+    down.
+  </summary>
+</histogram>
+
 <histogram name="Power.LengthOfSession" units="seconds">
   <summary>
     The length of time, in seconds, that a user spent in a single session.