Don't log security event to database on a read-only instance

Downloading the project source code on a Geo secondary
node tries to log a security event to the database
raising a  `PG::ReadOnlySqlTransaction` error.

app/services/audit_event_service.rb

@@ -44,6 +44,8 @@ class AuditEventService
   end
 
   def log_security_event_to_database
+    return if Gitlab::Database.read_only?
+
     SecurityEvent.create(base_payload.merge(details: @details))
   end
 end

ee/spec/services/audit_event_service_spec.rb

@@ -85,6 +85,16 @@ describe AuditEventService do
       it 'creates an event' do
         expect { service.security_event }.to change(SecurityEvent, :count).by(1)
       end
+
+      context 'on a read-only instance' do
+        before do
+          allow(Gitlab::Database).to receive(:read_only?).and_return(true)
+        end
+
+        it 'does not create an event' do
+          expect { service.security_event }.not_to change(SecurityEvent, :count)
+        end
+      end
     end
   end