public ServerBuilder workerGroup(int numThreads) {
        checkArgument(numThreads >= 0, "numThreads: %s (expected: >= 0)", numThreads);
        workerGroup(EventLoopGroups.newEventLoopGroup(numThreads), true);
        return this;
    }
Uses a newly created {@link EventLoopGroup} with the specified number of threads for
performing socket I/O and running {@link Service#serve(ServiceRequestContext, Request)}.
The worker {@link EventLoopGroup} will be shut down when the {@link Server} stops.

@param numThreads the number of event loop threads
core/src/main/java/com/linecorp/armeria/server/ServerBuilder.java