s:`MultipartEncoder` should only be responsible for preparing and
    streaming the data. For anyone who wishes to monitor it, they shouldn't be
    using that instance to manage that as well. Using this class, they can
    monitor an encoder and register a callback. The callback receives the
    instance of the monitor.

    To use this monitor, you construct your :class:`MultipartEncoder` as you
    normally would.

    .. code-block:: python

        from requests_toolbelt import (MultipartEncoder,
                                       MultipartEncoderMonitor)
        import requests

        def callback(encoder, bytes_read):
            # Do something with this information
            pass

        m = MultipartEncoder(fields={'field0': 'value0'})
        monitor = MultipartEncoderMonitor(m, callback)
        headers = {'Content-Type': montior.content_type}
        r = requests.post('https://httpbin.org/post', data=monitor,
                          headers=headers)

    Alternatively, if your use case is very simple, you can use the following
    pattern.

    .. code-block:: python

        from requests_toolbelt import MultipartEncoderMonitor
        import requests

        def callback(encoder, bytes_read):
            # Do something with this information
            pass

        monitor = MultipartEncoderMonitor.from_fields(
            fields={'field0': 'value0'}, callback
            )
        headers = {'Content-Type': montior.content_type}
        r = requests.post('https://httpbin.org/post', data=monitor,
                          headers=headers)

    Nc