add UMA metrics for voice interaction entries.

ash/shelf/app_list_button.cc

Index: ash/shelf/app_list_button.cc
diff --git a/ash/shelf/app_list_button.cc b/ash/shelf/app_list_button.cc
index e1fd34e35271883f0fc945b7a3123e26878bb19e..a8131d3d535cecfba581cca610c85b551d811a3c 100644
--- a/ash/shelf/app_list_button.cc
+++ b/ash/shelf/app_list_button.cc
@@ -17,6 +17,7 @@
 #include "ash/system/tray/tray_popup_utils.h"
 #include "base/command_line.h"
 #include "base/memory/ptr_util.h"
+#include "base/metrics/histogram_macros.h"
 #include "chromeos/chromeos_switches.h"
 #include "ui/accessibility/ax_node_data.h"
 #include "ui/app_list/presenter/app_list.h"
@@ -95,6 +96,9 @@ void AppListButton::OnGestureEvent(ui::GestureEvent* event) {
       break;
     case ui::ET_GESTURE_LONG_PRESS:
       if (chromeos::switches::IsVoiceInteractionEnabled()) {
+        UMA_HISTOGRAM_BOOLEAN(
+            "Shelf.Shortcuts.CrosVoiceInteractionEntryAppListButtonLongPress",
+            true);
         Shell::Get()->app_list()->StartVoiceInteractionSession();
         event->SetHandled();
       } else {