@Nullable
    default Long defaultServerConnectionDrainDurationMicros() {
        return null;
    }
Returns the default server-side graceful connection shutdown drain duration in microseconds.
If the value of this flag is greater than {@code 0}, a connection shutdown will have a drain period
when client will be notified about the shutdown, but in flight requests will still be accepted.

<p>The default value of this flag is
{@value DefaultFlagsProvider#DEFAULT_SERVER_CONNECTION_DRAIN_DURATION_MICROS}.
Specify the {@code -Dcom.linecorp.armeria.defaultServerConnectionDrainDurationMicros=<long>}
JVM option to override the default value.

<p>
At the beginning of the drain period server signals the clients that the connection shutdown is imminent
but still accepts in flight requests.
After the drain period end server stops accepting new requests.
</p>

<p>
Note that HTTP/1 doesn't support draining as described here, so for HTTP/1 drain period microseconds
is always {@code 0}, which means the connection will be closed immediately as soon as
the current in-progress request is handled.
</p>

@see ServerBuilder#connectionDrainDuration(Duration)
@see ServerBuilder#connectionDrainDurationMicros(long)
core/src/main/java/com/linecorp/armeria/common/FlagsProvider.java