public static EventLoopGroup workerGroup() {
        return WORKER_GROUP;
    }
Returns the common worker {@link EventLoopGroup} which is used when
{@link ServerBuilder#workerGroup(EventLoopGroup, boolean)} or
{@link ClientFactoryBuilder#workerGroup(EventLoopGroup, boolean)} is not specified.
core/src/main/java/com/linecorp/armeria/common/CommonPools.java