Instrument cached collection renders.

Augments the collection caching with some instrumentation that's logged.

For collections that have been cached like:

```ruby
<%= render partial: 'notifications/notification', collection: @notifications, cached: true %>
```

We'll output a line showing how many cache hits we had when rendering it:

```
  Rendered collection of notifications/_notification.html.erb [0 / 100 cache hits] (3396.5ms)
```

1 files changed, 17 insertions, 1 deletions

diff --git a/actionview/lib/action_view/log_subscriber.rb b/actionview/lib/action_view/log_subscriber.rb
index 9047dbdd85..aa38db2a3a 100644
--- a/actionview/lib/action_view/log_subscriber.rb
+++ b/actionview/lib/action_view/log_subscriber.rb
@@ -20,7 +20,15 @@ module ActionView
       end
     end
     alias :render_partial :render_template
-    alias :render_collection :render_template
+
+    def render_collection(event)
+      identifier = event.payload[:identifier] || 'templates'
+
+      info do
+        "  Rendered collection of #{from_rails_root(identifier)}" \
+        " #{render_count(event.payload)} (#{event.duration.round(1)}ms)"
+      end
+    end
 
     def logger
       ActionView::Base.logger
@@ -38,6 +46,14 @@ module ActionView
     def rails_root
       @root ||= "#{Rails.root}/"
     end
+
+    def render_count(payload)
+      if payload[:cache_hits]
+        "[#{payload[:cache_hits]} / #{payload[:count]} cache hits]"
+      else
+        "[#{payload[:count]} times]"
+      end
+    end
   end
 end