**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

airflow/www/views.py
"""Retrieve logs including metadata."""
        dag_id = request.args.get('dag_id')
        task_id = request.args.get('task_id')
        execution_date = request.args.get('execution_date')
        try_number = request.args.get('try_number', type=int)
        metadata = request.args.get('metadata')
        metadata = json.loads(metadata)
        response_format = request.args.get('format', 'json')
        
        if not metadata:
            metadata = {}
        
        try:
            execution_date = timezone.parse(execution_date)
        except ValueError: