public static long defaultClientHttp2GracefulShutdownTimeoutMillis() {
        return DEFAULT_CLIENT_HTTP2_GRACEFUL_SHUTDOWN_TIMEOUT_MILLIS;
    }
Returns the default client-side graceful connection shutdown timeout in milliseconds.
{@code 0} disables the timeout and closes the connection immediately after sending a GOAWAY frame.

<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.
After the drain period end client will close all the connections.
</p>

@see ClientFactoryBuilder#http2GracefulShutdownTimeout(Duration)
@see ClientFactoryBuilder#http2GracefulShutdownTimeoutMillis(long)
core/src/main/java/com/linecorp/armeria/common/Flags.java