create_custom_metric() shouldn't send "name"

The sample code for creating a custom metric ([create_custom_metric()](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/monitoring/api/v3/custom_metric.py#L62)) includes the `name` field in the request body. It shouldn't. The service ignores this field in the request (but includes it in the response).
