Add merge request data to the progress information on Milestones pages.

Fixes #13980.

CHANGELOG

@@ -23,6 +23,7 @@ v 8.9.0 (unreleased)
   - Added descriptions to notification settings dropdown
   - Improve note validation to prevent errors when creating invalid note via API
   - Reduce number of fog gem dependencies
+  - Add number of merge requests for a given milestone to the milestones view.
   - Implement a fair usage of shared runners
   - Remove project notification settings associated with deleted projects
   - Fix 404 page when viewing TODOs that contain milestones or labels in different projects

app/views/shared/milestones/_summary.html.haml

@@ -10,6 +10,13 @@
       open and
       %strong= milestone.issues_visible_to_user(current_user).closed.size
       closed
+    %strong= milestone.merge_requests.size
+    merge requests:
+    %span.milestone-stat
+      %strong= milestone.merge_requests.opened.size
+      open and
+      %strong= milestone.merge_requests.merged.size
+      merged
     %span.milestone-stat
       %strong== #{milestone.percent_complete(current_user)}%
       complete