Log structured message when LFS object is auto-linked from parent

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75972 added a
feature to auto-link uploaded LFS files from a push to a fork project.

To aid in the feature rollout in
https://gitlab.com/gitlab-org/gitlab/-/issues/348243, add a structured
log message.

Changelog: added

app/controllers/repositories/lfs_api_controller.rb

@@ -172,6 +172,14 @@ module Repositories
       return unless lfs_object
 
       LfsObjectsProject.link_to_project!(lfs_object, project)
+
+      Gitlab::AppJsonLogger.info(message: "LFS object auto-linked to forked project",
+        lfs_object_oid: lfs_object.oid,
+        lfs_object_size: lfs_object.size,
+        source_project_id: project.fork_source.id,
+        source_project_path: project.fork_source.full_path,
+        target_project_id: project.project_id,
+        target_project_path: project.full_path)
     end
   end
 end

config/feature_flags/development/lfs_auto_link_fork_source.yml

 ---
 name: lfs_auto_link_fork_source
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75972
-rollout_issue_url:
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/348243
 milestone: '14.6'
 type: development
 group: group::source code

spec/requests/lfs_http_spec.rb

@@ -532,6 +532,14 @@ RSpec.describe 'Git LFS API and storage' do
                     end
 
                     it 'links existing LFS objects to other project' do
+                      expect(Gitlab::AppJsonLogger).to receive(:info).with(
+                        message: "LFS object auto-linked to forked project",
+                        lfs_object_oid: lfs_object.oid,
+                        lfs_object_size: lfs_object.size,
+                        source_project_id: other_project.id,
+                        source_project_path: other_project.full_path,
+                        target_project_id: project.id,
+                        target_project_path: project.full_path).and_call_original
                       expect(json_response['objects']).to be_kind_of(Array)
                       expect(json_response['objects'].first).to include(sample_object)
                       expect(json_response['objects'].first).not_to have_key('actions')