Fix analytics tracking for new merge request notes

When a new note is created for a merge request with no associated
metrics, Sidekiq will error out with `NoMethodError: undefined method
'first_comment_at' for nil:NilClass`.

This commit ensures a merge request metrics exists before attempting to
update it. There is still an outstanding question of why no such metrics
were created, but that can happen for older merge requests.

Closes https://gitlab.com/gitlab-org/gitlab/issues/197464

changelogs/unreleased/sh-fix-issue-197464.yml

+---
+title: Fix analytics tracking for new merge request notes
+merge_request: 23273
+author:
+type: fixed

ee/lib/analytics/refresh_comments_data.rb

@@ -22,9 +22,11 @@ module Analytics
 
     def execute(force: false)
       merge_requests.each do |mr|
-        next if !force && mr.metrics.first_comment_at
+        metrics = mr.ensure_metrics
 
-        mr.metrics.update!(first_comment_at: ProductivityCalculator.new(mr).first_comment_at)
+        next if !force && metrics.first_comment_at
+
+        metrics.update!(first_comment_at: ProductivityCalculator.new(mr).first_comment_at)
       end
     end
 

ee/spec/lib/analytics/refresh_comments_data_spec.rb

@@ -25,6 +25,19 @@ describe Analytics::RefreshCommentsData do
         end.to change { noteable.metrics.first_comment_at }.from(nil).to(be_like_time(note.created_at))
       end
 
+      context 'when no merge request metric is present' do
+        before do
+          noteable.metrics.destroy
+          noteable.reload
+        end
+
+        it 'creates a new metric and updates the first_comment_at' do
+          subject.execute
+
+          expect(noteable.metrics.reload.first_comment_at).to(be_like_time(note.created_at))
+        end
+      end
+
       context 'and first_comment_at is already filled' do
         before do
           noteable.metrics.update(first_comment_at: 3.days.ago.beginning_of_day)