chromeos: Document Power.ConnectedChargingPorts histogram.

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 579764af88e3d65748a972badeec900e0ee1eb52..42d96199640559533bc2d1ddc5d4671a6e78e083 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -48662,6 +48662,22 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Power.ConnectedChargingPorts"
+    enum="PowerConnectedChargingPorts">
+  <owner>bleung@chromium.org</owner>
+  <owner>derat@chromium.org</owner>
+  <summary>
+    Connected charging ports on Chrome OS. A sample is reported every time that
+    the power manager polls sysfs (typically every 30 seconds). Ordinals are
+    assigned based on the lexicographical ordering of power supply names from
+    sysfs and have no implied correspondence with ports' physical locations. For
+    example, with ports 'CROS_USB_PD_CHARGER0' and 'CROS_USB_PD_CHARGER1',
+    'first' refers to the former and 'second' to the latter. To determine a
+    port's physical location, see the powerd charging_ports pref in the device's
+    overlay.
+  </summary>
+</histogram>
+
 <histogram name="Power.DarkResumeWakeDurationMs" units="ms">
   <owner>chirantan@chromium.org</owner>
   <summary>
@@ -100254,6 +100270,17 @@ value.
   <int value="4" label="Safe Spring Charger"/>
 </enum>
 
+<enum name="PowerConnectedChargingPorts" type="int">
+  <summary>
+    Connected charging ports on Chrome OS, as reported by the kernel.
+  </summary>
+  <int value="0" label="No ports connected"/>
+  <int value="1" label="First port connected"/>
+  <int value="2" label="Second port connected"/>
+  <int value="3" label="First and second ports connected"/>
+  <int value="4" label="More than two ports exist"/>
+</enum>
+
 <enum name="PowerSupplyType" type="int">
   <summary>
     The type of power supply connected to a Chrome OS system, as reported by the