@Nullable
    default Long defaultClientHttp2GracefulShutdownTimeoutMillis() {
        return null;
    }
Returns the default client-side graceful connection shutdown timeout in microseconds.

<p>Note that this flag has no effect if a user specified the value explicitly via
{@link ClientFactoryBuilder#http2GracefulShutdownTimeoutMillis(long)}.

<p>The default value of this flag is
{@value DefaultFlagsProvider#DEFAULT_CLIENT_HTTP2_GRACEFUL_SHUTDOWN_TIMEOUT_MILLIS}.
Specify the {@code -Dcom.linecorp.armeria.defaultClientHttp2GracefulShutdownTimeoutMillis=<long>}
JVM option to override the default value. {@code 0} disables the graceful shutdown.
core/src/main/java/com/linecorp/armeria/common/FlagsProvider.java