How does one create an index on a vertex label from Java?  I want to speed up queries that retrieve or count the vertices with a  particular label, e.g. g.V().hasLabel("foo").count().next().  In Gremlin-Groovy, I think you can use getPropertyKey(T.label) to reference the key that represents a label and pass that to addKey, but this does not work in Java because getPropertyKey expects a String and T.label is an enum.  What's the right way to do this?