public VirtualHostBuilder blockingTaskExecutor(int numThreads) {
        checkArgument(numThreads >= 0, "numThreads: %s (expected: >= 0)", numThreads);
        final BlockingTaskExecutor executor = BlockingTaskExecutor.builder()
                                                                  .numThreads(numThreads)
                                                                  .build();
        return blockingTaskExecutor(executor, true);
    }
Uses a newly created {@link BlockingTaskExecutor} with the specified number of threads dedicated to
the execution of blocking tasks or invocations.
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