private void registerMetrics(final String engineName) {
        final GremlinScriptEngine engine = gremlinExecutor.getScriptEngineManager().getEngineByName(engineName);
        MetricManager.INSTANCE.registerGremlinScriptEngineMetrics(engine, engineName, "sessionless", "class-cache");
    }
Create a new object from {@link Settings} where thread pools are externally assigned. Note that if the
{@code scheduleExecutorServiceClass} is set to {@code null} it will be created via
{@link Executors#newScheduledThreadPool(int, ThreadFactory)}.  If either of the {@link ExecutorService}
instances are supplied, the {@link Settings#gremlinPool} value will be ignored for the pool size.
gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/util/ServerGremlinExecutor.java