public String getIdPropertyKey() {
        return idPropertyKey;
    }
Provides a degree of control over element identifier assignment as some graphs don't provide that feature. This
strategy provides for identifier assignment by enabling users to utilize vertex and edge indices under the hood,
thus simulating that capability.
<p/>
By default, when an identifier is not supplied by the user, newly generated identifiers are {@link UUID} objects.
This behavior can be overridden by setting the {@link Builder#idMaker(Supplier)}.
<p/>
Unless otherwise specified the identifier is stored in the {@code __id} property.  This can be changed by setting
the {@link Builder#idPropertyKey(String)}

@author Marko A. Rodriguez (http://markorodriguez.com)
@author Stephen Mallette (http://stephen.genoprime.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/strategy/decoration/ElementIdStrategy.java