'Opens a Stream that the application can write the contents of the
file to.
The user must specify the filename, and can choose to add any
additional information in the metadata field of the file document or
modify the chunk size.
For example::
my_db = MongoClient().test
fs = GridFSBucket(my_db)
grid_in, file_id = fs.open_upload_stream(
"test_file", chunk_size_bytes=4,
metadata={"contentType": "text/plain"})
grid_in.write("data I want to store!")
grid_in.close()  # uploaded on close
Returns an instance of :class:`~gridfs.grid_file.GridIn`.
Raises :exc:`~gridfs.errors.NoFile` if no such version of
that file exists.
Raises :exc:`~ValueError` if `filename` is not a string.
:Parameters:
- `filename`: The name of the file to upload.
- `chunk_size_bytes` (options): The number of bytes per chunk of this
file. Defaults to the chunk_size_bytes in :class:`GridFSBucket`.
- `metadata` (optional): User data for the \'metadata\' field of the
files collection document. If not provided the metadata field will
be omitted from the files collection document.'

def open_upload_stream(self, filename, chunk_size_bytes=None, metadata=None):

    validate_string('filename', filename)
    opts = {'filename': filename, 'chunk_size': (chunk_size_bytes if (chunk_size_bytes is not None) else self._chunk_size_bytes)}
    if (metadata is not None):
        opts['metadata'] = metadata
    return GridIn(self._collection, **opts)
