Modify to use the level value of the event implementation instead of directly specifying the level

[syncpark]

Change to use the level value from event.level() instead of directly specifying the level.
Like following reference, the level is defined in DnsCovertChannel implementation already.

review-database/src/event.rs

Lines 969 to 981 in dc3127a

	pub fn count_level(
	&self,
	counter: &mut HashMap<NonZeroU8, usize>,
	locator: Option<Arc<Mutex<ip2location::DB>>>,
	filter: &EventFilter,
	) -> Result<()> {
	let mut level = None;
	match self {
	Event::DnsCovertChannel(event) => {
	if event.matches(locator, filter)?.0 {
	level = Some(MEDIUM);
	}
	}

review-database/src/event/dns.rs

Lines 138 to 140 in dc3127a

	fn level(&self) -> NonZeroU8 {
	MEDIUM
	}