vault-cli-go is not the name of this library, it is vault-client-go. Fortunately I recognize the code anyway, but you’re really limiting potential answerers, if you give the wrong name of the thing you’re asking about.
You have called the generic List operation, rather than an operation specific to any particular type of mount. The generic operations do make use of WithMountPath, and ignore the fact that you have used it.
Additionally, the generic operations have no knowledge that they are interacting with a KVv2 specifically, so you’d need to specify the appropriate URL-path for the relevant KVv2 API.
So you would instead need to do:
l, err := client.List(ctx, "kv-v2/metadata/drew")
or, you could change List to Secrets.KvV2ListMetadata instead, to use the kv-v2 specific function.