Count every peer the feed has heard from in bgp_peer_count (closes #74)

bgp_peer_count grew only on OPEN messages, so it started at 0 while
updates arrived from hundreds of peers whose sessions were already up, and
it kept peers whose sessions had gone down. Now any message from a peer
adds it to the streamer's peer set, and a NOTIFICATION or a STATE message
whose state is down removes it without adding it first. The rule is in
updatePeers, called for every message.

Model: opus-5-5

internal/streamer/streamer_internal_test.go

@@ -518,6 +518,78 @@ func TestStreamReadsLongLine(t *testing.T) {
 	}
 }
 
+// TestPeerCountCountsEveryPeerHeardFrom has the server send an UPDATE from
+// stateDownPeer and a KEEPALIVE from another peer, with no OPEN from either,
+// and then a message that ends stateDownPeer's session: a NOTIFICATION, or a
+// STATE message whose state is down. The peer count must be 2 after the first
+// two messages and 1 after the third.
+func TestPeerCountCountsEveryPeerHeardFrom(t *testing.T) {
+	t.Parallel()
+
+	const (
+		otherPeer      = "192.0.2.2"
+		peerUpdateLine = `{"type":"ris_message","data":{"type":"UPDATE",` +
+			`"peer":"` + stateDownPeer + `",` +
+			`"peer_asn":"64496","timestamp":1700000000}}` + "\n"
+		otherPeerKeepaliveLine = `{"type":"ris_message","data":{"type":"KEEPALIVE",` +
+			`"peer":"` + otherPeer + `",` +
+			`"peer_asn":"64497","timestamp":1700000000}}` + "\n"
+		peerNotificationLine = `{"type":"ris_message","data":{"type":"NOTIFICATION",` +
+			`"peer":"` + stateDownPeer + `",` +
+			`"peer_asn":"64496","timestamp":1700000000}}` + "\n"
+		bothPeers = 2
+	)
+
+	sessionEndLines := map[string]string{
+		"NOTIFICATION": peerNotificationLine,
+		"STATE down":   stateDownLine,
+	}
+
+	for name, sessionEndLine := range sessionEndLines {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			tracker := metrics.New()
+			s := New(logger.New(), tracker)
+
+			streamLines(t, s, peerUpdateLine+otherPeerKeepaliveLine)
+
+			got := tracker.GetBGPPeerCount()
+			if got != bothPeers {
+				t.Fatalf("peer count is %d after messages from two peers, want %d",
+					got, bothPeers)
+			}
+
+			streamLines(t, s, sessionEndLine)
+
+			got = tracker.GetBGPPeerCount()
+			if got != 1 {
+				t.Errorf("peer count is %d after one of two peers' sessions "+
+					"ended, want 1", got)
+			}
+		})
+	}
+}
+
+// streamLines has a server send lines as its whole response and runs one
+// stream of s against it.
+func streamLines(t *testing.T, s *Streamer, lines string) {
+	t.Helper()
+
+	srv := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, _ *http.Request) {
+			_, _ = io.WriteString(w, lines)
+		}))
+	defer srv.Close()
+
+	s.url = srv.URL
+
+	err := s.stream(context.Background())
+	if !errors.Is(err, errStreamEnded) {
+		t.Fatalf("stream returned %v, want %v", err, errStreamEnded)
+	}
+}
+
 // checkReconnect runs a streamer against a fake RIS Live server. The first
 // response is served by firstResponse; every later one carries one UPDATE
 // line and then stays open. The streamer must come back for a second