Log when primary host for DB load balancing is marked offline

Changelog: changed

lib/gitlab/database/load_balancing/primary_host.rb

@@ -49,6 +49,11 @@ module Gitlab
         end
 
         def offline!
+          ::Gitlab::Database::LoadBalancing::Logger.warn(
+            event: :host_offline,
+            message: 'Marking primary host as offline'
+          )
+
           nil
         end
 

spec/lib/gitlab/database/load_balancing/primary_host_spec.rb

@@ -51,7 +51,11 @@ RSpec.describe Gitlab::Database::LoadBalancing::PrimaryHost do
   end
 
   describe '#offline!' do
-    it 'does nothing' do
+    it 'logs the event but does nothing else' do
+      expect(Gitlab::Database::LoadBalancing::Logger).to receive(:warn)
+        .with(hash_including(event: :host_offline))
+        .and_call_original
+
       expect(host.offline!).to be_nil
     end
   end