Flask logging with Foreman

I'm trying to set up a Heroku-ready Flask app, but I can't figure out how to turn on logging.

Without Foreman, I could create a helloworld app as described in the Flask tutorial:

from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    app.logger.debug('A value for debugging')
    app.logger.warning('A value for warning')
    return "Hello World!"

if __name__ == "__main__":
    app.run(debug=True)

start it like so:

python hello.py

and have logging in stdout.

When I follow the Heroku tutorial, however, there's no app.run line:

import os
from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello():
    app.logger.debug('A value for debugging')
    app.logger.warning('A value for warning')
    return 'Hello World!'

And so I can't figure out how to run in debug mode and/or get logging output:

foreman start -p 5000

Procfile:

web: gunicorn hello:app