**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
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)
        expected_filename = '{}/{}/{}/{}.log'.format(
            self.DAG_ID, self.TASK_ID, self.DEFAULT_DATE.isoformat(), try_number
        )
        content_disposition = response.headers.get('Content-Disposition')
        self.assertTrue(content_disposition.startswith('attachment'))
        self.assertTrue(expected_filename in content_disposition)
        self.assertEqual(200, response.status_code)
        self.assertIn('Log for testing.', response.data.decode('utf-8'))
    def test_get_logs_with_metadata_as_download_large_file(self):