Let bgp_peer_count drop to 0 (closes #92)

SetBGPPeerCount stored a count only when it was above 0, so when the last
peer the streamer knew of went down, bgp_peer_count kept its old value. It
now stores every count from 0 up.

Model: opus-5-5

internal/metrics/metrics.go

@@ -168,7 +168,7 @@ func (t *Tracker) RecordWithdrawal() {
 // SetBGPPeerCount updates the current BGP peer count
 func (t *Tracker) SetBGPPeerCount(count int) {
 	// BGP peer count is always small (< 1000), so int32 is safe
-	if count > 0 && count < 1<<31 {
+	if count >= 0 && count < 1<<31 {
 		t.bgpPeerCount.Store(int32(count))
 	}
 }