Reset the peering handler's dropped_paths figure after each log line (closes #116)

processPeerings replaced the path map every run but kept the count of
paths dropped while the map was full, so every later "Processed AS
peerings" line reported all drops since start next to that run's own
figures. The count is now set to 0 where the map is replaced, under the
same lock. A test fills the map, has one path dropped, runs the
processing and finds the count back at 0.

Model: opus-5-5

internal/routewatch/peeringhandler_internal_test.go

@@ -264,6 +264,36 @@ func TestPeeringHandlerCapDropsAndCounts(t *testing.T) {
 	}
 }
 
+// TestPeeringHandlerRunResetsTheDroppedCount fills the map, has the handler
+// drop one path, and runs the processing. The count of dropped paths is the
+// run's own, so it is 0 again after the run.
+func TestPeeringHandlerRunResetsTheDroppedCount(t *testing.T) {
+	t.Parallel()
+
+	h := newTestHandler(newMockStore())
+
+	now := time.Now().UTC()
+
+	// Paths of one AS carry no peering, so the run records nothing.
+	h.mu.Lock()
+	for asn := 1; len(h.asPaths) < maxTrackedPaths; asn++ {
+		h.asPaths[pathKey(t, asn)] = database.SeenTimes{First: now, Last: now}
+	}
+	h.mu.Unlock()
+
+	handle(h, now, testASNA, testASNB)
+
+	if _, dropped := snapshot(h); dropped != 1 {
+		t.Fatalf("want dropped count 1 with the map full, got %d", dropped)
+	}
+
+	h.ProcessPeeringsNow()
+
+	if _, dropped := snapshot(h); dropped != 0 {
+		t.Errorf("want dropped count 0 after a run, got %d", dropped)
+	}
+}
+
 // TestPeeringHandlerStoresTheEarliestAndLatestTimesOfANewPeering handles two
 // updates whose path carries a peering the database does not hold yet, the
 // second 5 seconds after the first, in one run into a real database. The