**Description**
In the trigger page, there is a text area to enter (optional) conf json. It would be great if a sample JSON can be provided programatically while defining DAG

**Use case / motivation**

This will improve usability of the UI that triggers a DAG

tests/www/test_views.py
with mock.patch("airflow.utils.log.file_task_handler.FileTaskHandler.read") as read_mock:
            first_return = ([[('default_log', '1st line')]], [{}])
            second_return = ([[('default_log', '2nd line')]], [{'end_of_log': False}])
            third_return = ([[('default_log', '3rd line')]], [{'end_of_log': True}])
            fourth_return = ([[('default_log', 'should never be read')]], [{'end_of_log': True}])
            read_mock.side_effect = [first_return, second_return, third_return, fourth_return]
            url_template = (
                "get_logs_with_metadata?dag_id={}&"
                "task_id={}&execution_date={}&"
                "try_number={}&metadata={}&format=file"
            )
            try_number = 1
            url = url_template.format(
                self.DAG_ID,
                self.TASK_ID,
                quote_plus(self.DEFAULT_DATE.isoformat()),
                try_number,
                json.dumps({}),
            )
            response = self.client.get(url)
            self.assertIn('1st line', response.data.decode('utf-8'))
            self.assertIn('2nd line', response.data.decode('utf-8'))
            self.assertIn('3rd line', response.data.decode('utf-8'))
            self.assertNotIn('should never be read', response.data.decode('utf-8'))
    def test_get_logs_with_metadata(self):