@Nullable
    default Integer numCommonWorkers(TransportType transportType) {
        return null;
    }
Returns the default number of {@linkplain CommonPools#workerGroup() common worker group} threads.
Note that this flag has no effect if a user specified the worker group explicitly via
{@link ServerBuilder#workerGroup(EventLoopGroup, boolean)} or
{@link ClientFactoryBuilder#workerGroup(EventLoopGroup, boolean)}.

<p>The default value of this flag is {@code 2 * <numCpuCores>} for {@link TransportType#NIO},
{@link TransportType#EPOLL} and {@link TransportType#KQUEUE} and {@code <numCpuCores>} for
{@link TransportType#IO_URING}. Specify the {@code -Dcom.linecorp.armeria.numCommonWorkers=<integer>}
JVM option to override the default value.

@param transportType the {@link TransportType} that will be used for I/O
core/src/main/java/com/linecorp/armeria/common/FlagsProvider.java