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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_service_account_token(request, service_account='default'):
 """Get the OAuth 2.0 access token for a service account. Args: request (google.auth.transport.Request):
 A callable used to make HTTP requests. service_account (str):
 The string 'default' or a service account email address. The determines which service account for which to acquire an access token. Returns: Union[str, datetime]: The access token and its expiration. Raises: google.auth.exceptions.TransportError: if an error occurred while retrieving metadata. """

    token_json = get(
        request,
        'instance/service-accounts/{0}/token'.format(service_account))
    token_expiry = _helpers.utcnow() + datetime.timedelta(
        seconds=token_json['expires_in'])
    return token_json['access_token'], token_expiry