Add MergeRequestDiff#lines_count convenience method

app/models/merge_request_diff.rb

@@ -196,6 +196,12 @@ class MergeRequestDiff < ApplicationRecord
     real_size.presence || raw_diffs.size
   end
 
+  def lines_count
+    strong_memoize(:lines_count) do
+      diffs.diff_files.sum(&:line_count)
+    end
+  end
+
   def raw_diffs(options = {})
     if options[:ignore_whitespace_change]
       @diffs_no_whitespace ||= compare.diffs(options)

spec/lib/gitlab/import_export/safe_model_attributes.yml

@@ -235,6 +235,12 @@ MergeRequest::Metrics:
 - latest_build_started_at
 - latest_build_finished_at
 - first_deployed_to_production_at
+- first_comment_at
+- first_commit_at
+- last_commit_at
+- diff_size
+- modified_paths_size
+- commits_count
 Ci::Pipeline:
 - id
 - project_id

spec/models/merge_request_diff_spec.rb

@@ -484,4 +484,12 @@ describe MergeRequestDiff do
       end
     end
   end
+
+  describe '#lines_count' do
+    subject { diff_with_commits }
+
+    it 'returns sum of all changed lines count in diff files' do
+      expect(subject.lines_count).to eq 109
+    end
+  end
 end