@Override
    public long getDuration(final TimeUnit unit) {
        return unit.convert(this.durationNs, SOURCE_UNIT);
    }
A {@link Metrics} implementation that cannot be modified.

@author Bob Briody (http://bobbriody.com)
@author Stephen Mallette (http://stephen.genoprime.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/util/ImmutableMetrics.java