Fix Gitaly N+1 queries in related merge requests API

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9661, we
removed the `subscribed` field from API endpoints that needed to return
a list of issues or merge requests when to avoid full Markdown
processing.

However, we introduced a performance regression in GitLab 11.10
(https://gitlab.com/gitlab-org/gitlab-foss/merge_requests/26367) when we
converted `MergeRequestBasic` to `MergeRequest` in order to expose the
head pipeline in the
/api/:version/projects/:id/issues/:issue_iid/related_merge_requests
endpoint.

To avoid this overhead, we now omit the `subscribed` field in this API
endpoint.

Closes https://gitlab.com/gitlab-org/gitlab/issues/32948

changelogs/unreleased/sh-fix-gitaly-nplus-one-issues-related-mrs.yml

+---
+title: Fix Gitaly N+1 queries in related merge requests API
+merge_request: 17850
+author:
+type: performance

lib/api/entities.rb

@@ -771,7 +771,7 @@ module API
     end
 
     class MergeRequest < MergeRequestBasic
-      expose :subscribed do |merge_request, options|
+      expose :subscribed, if: -> (_, options) { options.fetch(:include_subscribed, true) } do |merge_request, options|
         merge_request.subscribed?(options[:current_user], options[:project])
       end
 

lib/api/issues.rb

@@ -343,7 +343,8 @@ module API
         present paginate(::Kaminari.paginate_array(merge_requests)),
           with: Entities::MergeRequest,
           current_user: current_user,
-          project: user_project
+          project: user_project,
+          include_subscribed: false
       end
 
       desc 'List merge requests closing issue' do

spec/requests/api/issues/get_project_issues_spec.rb

@@ -736,6 +736,8 @@ describe API::Issues do
         get_related_merge_requests(project.id, issue.iid)
 
         expect_paginated_array_response(related_mr.id)
+        expect(response).to have_gitlab_http_status(200)
+        expect(json_response.last).not_to have_key('subscribed')
       end
 
       it 'renders 404 if project is not visible' do