[silences] Don't merge expired silences

If they're expired, they should be cleaned up on
the next GC cycle, but merging them in means that
they'll probably be gossip'd continually between
the cluster members.

Signed-off-by: stuart nelson <stuartnelson3@gmail.com>

silence/silence.go

@@ -717,7 +717,12 @@ func (s *Silences) Merge(b []byte) error {
 	s.mtx.Lock()
 	defer s.mtx.Unlock()
 
+	now := s.now()
+
 	for _, e := range st {
+		if e.ExpiresAt.Before(now) {
+			continue
+		}
 		if merged := s.st.merge(e); merged && !cluster.OversizedMessage(b) {
 			// If this is the first we've seen the message and it's
 			// not oversized, gossip it to other nodes. We don't