@Override
    public long getDuration(final TimeUnit unit) {
        return unit.convert(this.totalStepDuration, MutableMetrics.SOURCE_UNIT);
    }
This is only a convenient constructor needed for GraphSON deserialization.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/util/DefaultTraversalMetrics.java