use extend self pattern instead of defining self.xxx methods all over the place

app/models/report/filter.rb

@@ -1,15 +1,17 @@
 require "set"
 
 module Report::Filter
-  def self.all
+  extend self
+
+  def all
     @all ||= Set[]
   end
 
-  def self.all_grouped
+  def all_grouped
     all.group_by { |f| f.applies_for }.to_a.sort { |a,b| a.first.to_s <=> b.first.to_s }
   end
 
-  def self.from_hash
+  def from_hash
     raise NotImplementedError
   end
 end