@UnstableApi
    public VirtualHostBuilder serviceWorkerGroup(int numThreads) {
        final EventLoopGroup workerGroup = EventLoopGroups.newEventLoopGroup(numThreads);
        return serviceWorkerGroup(workerGroup, true);
    }
Uses a newly created {@link EventLoopGroup} with the specified number of threads dedicated to
the execution of services' methods.
The worker {@link EventLoopGroup} will be shut down when the {@link Server} stops.

@param numThreads the number of threads in the executor
core/src/main/java/com/linecorp/armeria/server/VirtualHostBuilder.java