public Builder partitionKey(final String partitionKey) {
            this.partitionKey = partitionKey;
            return this;
        }
Specifies the partition key name.  This is the property key that contains the partition value. It
may a good choice to index on this key in certain cases (in graphs that support such things). This
value must be specified for the {@code PartitionStrategy} to be constructed properly.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/strategy/decoration/PartitionStrategy.java