Added generic Sequencer Unit to have a statistics diff with a custom key.

lib/sequencer/unit/import/common/model/statistics/diff/custom_key.rb

@@ -0,0 +1,33 @@
+require 'sequencer/unit/import/common/model/statistics/mixin/action_diff'
+
+class Sequencer
+  class Unit
+    module Import
+      module Common
+        module Model
+          module Statistics
+            module Diff
+              class CustomKey < Sequencer::Unit::Base
+                include ::Sequencer::Unit::Import::Common::Model::Statistics::Mixin::ActionDiff
+
+                def process
+                  state.provide(:statistics_diff) do
+                    {
+                      key => diff,
+                    }
+                  end
+                end
+
+                private
+
+                def key
+                  raise "Missing implementation of method 'key' for class #{self.class.name}"
+                end
+              end
+            end
+          end
+        end
+      end
+    end
+  end
+end