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)
```

3 files changed, 21 insertions, 16 deletions

diff --git a/actionview/lib/action_view/renderer/abstract_renderer.rb b/actionview/lib/action_view/renderer/abstract_renderer.rb
index aa77a77acf..23e672a95f 100644
--- a/actionview/lib/action_view/renderer/abstract_renderer.rb
+++ b/actionview/lib/action_view/renderer/abstract_renderer.rb
@@ -35,8 +35,12 @@ module ActionView
       end
     end
 
-    def instrument(name, options={})
-      ActiveSupport::Notifications.instrument("render_#{name}.action_view", options){ yield }
+    def instrument(name, **options)
+      options[:identifier] ||= (@template && @template.identifier) || @path
+
+      ActiveSupport::Notifications.instrument("render_#{name}.action_view", options) do |payload|
+        yield payload
+      end
     end
 
     def prepend_formats(formats)
diff --git a/actionview/lib/action_view/renderer/partial_renderer.rb b/actionview/lib/action_view/renderer/partial_renderer.rb
index bdbf03191a..514804b08e 100644
--- a/actionview/lib/action_view/renderer/partial_renderer.rb
+++ b/actionview/lib/action_view/renderer/partial_renderer.rb
@@ -294,7 +294,7 @@ module ActionView
 
     def render(context, options, block)
       setup(context, options, block)
-      identifier = (@template = find_partial) ? @template.identifier : @path
+      @template = find_partial
 
       @lookup_context.rendered_format ||= begin
         if @template && @template.formats.present?
@@ -305,11 +305,9 @@ module ActionView
       end
 
       if @collection
-        instrument(:collection, :identifier => identifier || "collection", :count => @collection.size) do
-          render_collection
-        end
+        render_collection
       else
-        instrument(:partial, :identifier => identifier) do
+        instrument(:partial) do
           render_partial
         end
       end
@@ -318,15 +316,17 @@ module ActionView
     private
 
     def render_collection
-      return nil if @collection.blank?
+      instrument(:collection, count: @collection.size) do |payload|
+        return nil if @collection.blank?
 
-      if @options.key?(:spacer_template)
-        spacer = find_template(@options[:spacer_template], @locals.keys).render(@view, @locals)
-      end
+        if @options.key?(:spacer_template)
+          spacer = find_template(@options[:spacer_template], @locals.keys).render(@view, @locals)
+        end
 
-      cache_collection_render do
-        @template ? collection_with_template : collection_without_template
-      end.join(spacer).html_safe
+        cache_collection_render(payload) do
+          @template ? collection_with_template : collection_without_template
+        end.join(spacer).html_safe
+      end
     end
 
     def render_partial
diff --git a/actionview/lib/action_view/renderer/partial_renderer/collection_caching.rb b/actionview/lib/action_view/renderer/partial_renderer/collection_caching.rb
index debfcae870..f7deba94ce 100644
--- a/actionview/lib/action_view/renderer/partial_renderer/collection_caching.rb
+++ b/actionview/lib/action_view/renderer/partial_renderer/collection_caching.rb
@@ -9,11 +9,12 @@ module ActionView
     end
 
     private
-      def cache_collection_render
+      def cache_collection_render(instrumentation_payload)
         return yield unless @options[:cached]
 
         keyed_collection = collection_by_cache_keys
-        cached_partials = collection_cache.read_multi(*keyed_collection.keys)
+        cached_partials  = collection_cache.read_multi(*keyed_collection.keys)
+        instrumentation_payload[:cache_hits] = cached_partials.size
 
         @collection = keyed_collection.reject { |key, _| cached_partials.key?(key) }.values
         rendered_partials = @collection.empty? ? [] : yield