The documentation and some code comments think there should be an undelete for bigtable instances:

https://github.com/GoogleCloudPlatform/gcloud-python/blob/e63b1d8bd24a826c28865a21766ee7537ce87285/gcloud/bigtable/instance.py#L283

https://googlecloudplatform.github.io/gcloud-python/stable/bigtable-instance.html

but AFAICT looking at code it appears to be unimplemented.