Log extra metadata about searches

Logging these extra fields as top level keys will ensure that we can
query/filter them in Kibana which will help us to answer questions about
common search terms or understand usage of search within individual
groups. Currently these query params are already logged but they are
nested keys they are not indexed and cannot be filtered in any way in
Kibana.

This change adds to the [existing logic in ApplicationController](
t https://gitlab.com/gitlab-org/gitlab/-/blob/452043cb89801feec2609b83a4ab6a25bfc99c08/app/controllers/application_controller.rb#L148
)
which in term relies on the [`append_info_to_payload`](
https://github.com/rails/rails/blob/fbe2433be6e052a1acac63c7faf287c52ed3c5ba/actionpack/lib/action_controller/metal/instrumentation.rb#L89
) being invoked after a request.

https://gitlab.com/gitlab-org/gitlab/-/issues/227884

app/controllers/search_controller.rb

@@ -114,4 +114,15 @@ class SearchController < ApplicationController
 
     Gitlab::UsageDataCounters::SearchCounter.count(:navbar_searches)
   end
+
+  def append_info_to_payload(payload)
+    super
+
+    # Merging to :metadata will ensure these are logged as top level keys
+    payload[:metadata] || {}
+    payload[:metadata]['meta.search.group_id'] = params[:group_id]
+    payload[:metadata]['meta.search.project_id'] = params[:project_id]
+    payload[:metadata]['meta.search.search'] = params[:search]
+    payload[:metadata]['meta.search.scope'] = params[:scope]
+  end
 end

spec/controllers/search_controller_spec.rb

@@ -216,4 +216,23 @@ RSpec.describe SearchController do
     it_behaves_like 'when the user cannot read cross project', :autocomplete, { term: 'hello' }
     it_behaves_like 'with external authorization service enabled', :autocomplete, { term: 'hello' }
   end
+
+  describe '#append_info_to_payload' do
+    it 'appends search metadata for logging' do
+      last_payload = nil
+      original_append_info_to_payload = controller.method(:append_info_to_payload)
+
+      expect(controller).to receive(:append_info_to_payload) do |payload|
+        original_append_info_to_payload.call(payload)
+        last_payload = payload
+      end
+
+      get :show, params: { scope: 'issues', search: 'hello world', group_id: '123', project_id: '456' }
+
+      expect(last_payload[:metadata]['meta.search.group_id']).to eq('123')
+      expect(last_payload[:metadata]['meta.search.project_id']).to eq('456')
+      expect(last_payload[:metadata]['meta.search.search']).to eq('hello world')
+      expect(last_payload[:metadata]['meta.search.scope']).to eq('issues')
+    end
+  end
 end