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..074df27844a9ff4cf24f93bddac352527914e4c8 100644
--- a/ash/shelf/app_list_button.cc
+++ b/ash/shelf/app_list_button.cc
@@ -7,6 +7,7 @@
 #include <memory>
 #include <utility>
 
+#include "ash/metrics/voice_interaction_metrics.h"
 #include "ash/public/cpp/shelf_types.h"
 #include "ash/shelf/ink_drop_button_listener.h"
 #include "ash/shelf/shelf_constants.h"
@@ -17,6 +18,7 @@
 #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 +97,10 @@ void AppListButton::OnGestureEvent(ui::GestureEvent* event) {
       break;
     case ui::ET_GESTURE_LONG_PRESS:
       if (chromeos::switches::IsVoiceInteractionEnabled()) {
+        UMA_HISTOGRAM_ENUMERATION(
+            "VoiceInteraction.EntryPoints",
+            ash::VoiceInteractionEntry::LONG_PRESS_APP_LIST_BUTTON,
+            ash::VoiceInteractionEntry::COUNT);
         Shell::Get()->app_list()->StartVoiceInteractionSession();
         event->SetHandled();
       } else {