@Nullable
    default Long defaultClientIdleTimeoutMillis() {
        return null;
    }
Returns the default client-side idle timeout of a connection for keep-alive in milliseconds.
Note that this flag has no effect if a user specified the value explicitly via
{@link ClientFactoryBuilder#idleTimeout(Duration)}.

<p>This default value of this flag is {@value DefaultFlagsProvider#DEFAULT_CLIENT_IDLE_TIMEOUT_MILLIS}.
Specify the {@code -Dcom.linecorp.armeria.defaultClientIdleTimeoutMillis=<integer>} JVM option to
override the default value.
core/src/main/java/com/linecorp/armeria/common/FlagsProvider.java