@Override
    public String toString() {
        // Build a pretty table of metrics data.

        // Append headers
        final StringBuilder sb = new StringBuilder("Traversal Metrics")
                .append(System.lineSeparator())
                .append(String.format("%-50s %21s %11s %15s %8s", HEADERS));
        sb.append(System.lineSeparator());
        sb.append("=============================================================================================================");

        appendMetrics(this.positionIndexedMetrics.values(), sb, 0);

        // Append total duration
        sb.append(String.format("%n%50s %21s %11s %15.3f %8s",
                ">TOTAL", "-", "-", getDuration(TimeUnit.MICROSECONDS) / 1000.0, "-"));

        return sb.toString();
    }
The metrics have been computed and can no longer be modified.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/util/DefaultTraversalMetrics.java