This is not an official Google product
VMRegistry is a simple GRPC-based API around libvirt that allows to query VM details and to manage VM state.
You can find all the currently exposed APIs in proto/vmregistry.proto.
VMRegistry auth is based on JWT as provided by credstore. Consult credstore documentation on how to generate a token.
There's no RBAC at the moment, so anyone holding a valid token has full access to the vmregistry, possibly meaning a transitive root access to the host node via libvirt.