@Override
    public Long getCount(final String key) {
        if (!counts.containsKey(key)) {
            return null;
        }
        return counts.get(key).get();
    }
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