add multifile editor metrics

ee/lib/gitlab/metrics/multi_file_editor.rb

+module Gitlab
+  module Metrics
+    class MultiFileEditor
+      delegate :total, to: :commit_stats, prefix: :line_changes
+
+      METRIC_NAME = :multi_file_editor_usage
+
+      def initialize(project, current_user, commit)
+        @project, @current_user, @commit = project, current_user, @commit = commit
+      end
+
+      def record
+        ::Gitlab::Metrics.counter(
+            METRIC_NAME,
+            'Total number of commits using the multi-file web editor',
+            metric_labels)
+      end
+
+      private
+
+      def files_total
+        @commit.diffs.size
+      end
+
+      def hashed_project
+        Digest::SHA256.hexdigest("#{@project.id}-#{Rails.application.secrets.secret_key_base}")
+      end
+
+      def hashed_user
+        Digest::SHA256.hexdigest("#{@current_user.id}-#{Rails.application.secrets.secret_key_base}")
+      end
+
+      def commit_stats
+        @commit.stats
+      end
+
+      def metric_labels
+        {
+            project: hashed_project,
+            user: hashed_user,
+            line_changes: line_changes_total,
+            files_count: files_total
+        }
+      end
+    end
+  end
+end