Add BluetoothRemoteGATTDescriptor reporting for .readValue().

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 3df69e024281ff6e8feecfc6c7d3f2e20c903cc2..976e3161063cfe6be9948617d76340f2c8020844 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -5016,6 +5016,19 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Bluetooth.Web.Descriptor.ReadValue.Outcome"
+    enum="WebBluetoothGATTOperationOutcome">
+  <owner>jyasskin@chromium.org</owner>
+  <owner>ortuno@chromium.org</owner>
+  <owner>scheib@chromium.org</owner>
+  <summary>
+    Records the outcome of a call to descriptor.readValue(). Used to know what
+    types of errors users are seeing. The results will be used to determine how
+    common these errors are and if we need to provide better error messages to
+    the users.
+  </summary>
+</histogram>
+
 <histogram name="Bluetooth.Web.FunctionCall.Count" enum="WebBluetoothFunction">
   <owner>jyasskin@chromium.org</owner>
   <owner>ortuno@chromium.org</owner>