Added UMA stats for extensions events on addListener.

chrome/browser/extensions/event_router.cc

Index: chrome/browser/extensions/event_router.cc
diff --git a/chrome/browser/extensions/event_router.cc b/chrome/browser/extensions/event_router.cc
index df5b2f7b31e39475914e9f955aea02a1c2b5bac4..e7a556be86217544c8c51fb80798d0c4c8acd6ed 100644
--- a/chrome/browser/extensions/event_router.cc
+++ b/chrome/browser/extensions/event_router.cc
@@ -9,6 +9,7 @@
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/message_loop.h"
+#include "base/metrics/histogram.h"
 #include "base/stl_util.h"
 #include "base/values.h"
 #include "base/version.h"
@@ -30,6 +31,7 @@
 #include "chrome/common/chrome_notification_types.h"
 #include "chrome/common/chrome_switches.h"
 #include "chrome/common/extensions/api/extension_api.h"
+#include "chrome/common/extensions/api/generated_api.h"
 #include "chrome/common/extensions/extension.h"
 #include "chrome/common/extensions/extension_messages.h"
 #include "chrome/common/view_type.h"
@@ -186,6 +188,9 @@ EventRouter::EventRouter(Profile* profile, ExtensionPrefs* extension_prefs)
   registrar_.Add(this, chrome::NOTIFICATION_EXTENSION_INSTALLED,
                  content::Source<Profile>(profile_));
 
+  extensions::api::GeneratedEventHistogramMap::
+    AddAll(histogram_values_);
+
   // NULL in unit_tests.
   if (extension_prefs) {
     // Check if registered events are up-to-date. We need to do this before
@@ -203,6 +208,7 @@ void EventRouter::AddEventListener(const std::string& event_name,
                                    const std::string& extension_id) {
   listeners_.AddListener(scoped_ptr<EventListener>(new EventListener(
       event_name, extension_id, process, scoped_ptr<DictionaryValue>())));
+  RecordEventListenerAdded(event_name);

[koz (OOO until 15th September), 2013/03/13 01:00:27]

This might be trickier than it seems. Extensions / apps with lazy listeners will
repeatedly call AddEventListener() because they will be loaded and unloaded many
times. When a non-persistent extension is loaded it will add both a lazy
listener and a regular listener.

One way to solve this might be to distinguish between persistent and
non-persistent extensions and track listener adds for the former and lazy
listener adds for the later.

But even that has the problem that whenever the browser starts persistent
extensions will call AddEventListener() again and we will track it again. This
might not be too bad as we might reason that on average all persistent
extensions are going to be restarted roughly the same number of times, so that
can be factored out, but it's still quite dodgy...

We should clarify what we intend to find out from these statistics. If we want
to measure API usage there might be another way (eg: grep over the webstore)...

 }
 
 void EventRouter::RemoveEventListener(const std::string& event_name,
@@ -592,6 +598,11 @@ void EventRouter::DispatchPendingEvent(const linked_ptr<Event>& event,
   }
 }
 
+void EventRouter::RecordEventListenerAdded(const std::string& event_name) {
+  UMA_HISTOGRAM_ENUMERATION("Extensions.AddEventListener",
+      histogram_values_[event_name], extensions::events::ENUM_BOUNDARY);
+}
+
 void EventRouter::Observe(int type,
                           const content::NotificationSource& source,
                           const content::NotificationDetails& details) {