DEV: Introduce plugin API for getting stats (#24829)

Before, when needed to get stats in a plugin, we called Core classes directly. 
Introducing plugin API will decouple plugins from Core and give as more freedom 
in refactoring stats in Core. Without this API, I wasn't able to do all refactorings 
I wanted when working on d91456f.

lib/plugin/instance.rb

@@ -74,6 +74,11 @@ class Plugin::Instance
     @seed_fu_filter = filter
   end
 
+  # This method returns Core stats + stats registered by plugins
+  def self.stats
+    Stat.all_stats
+  end
+
   def self.find_all(parent_path)
     [].tap do |plugins|
       # also follows symlinks - http://stackoverflow.com/q/357754