Show route churn as a whole count per minute (closes #160)

The status page showed route churn per second as a one-minute moving
average, so a quiet feed gave a fraction such as 0.1. The average was
correct.

The metrics tracker now also counts the prefixes announced or withdrawn
in each clock minute. /api/v1/stats reports the count for the last full
minute as route_churn_per_min in stream, in place of
route_churn_per_sec. The status page shows it as "Route Churn/min".

The status and home pages round messages and updates per second to
whole numbers.

Model: opus-5-5

internal/metrics/metrics.go

@@ -34,7 +34,7 @@ type Tracker struct {
 	// Announcement/withdrawal metrics
 	announcementCounter metrics.Counter
 	withdrawalCounter   metrics.Counter
-	churnRate           metrics.Meter // combined announcements + withdrawals per second
+	churnPerMinute      minuteCounter // announced and withdrawn prefixes
 
 	// BGP peer tracking
 	bgpPeerCount atomic.Int32
@@ -56,7 +56,7 @@ func New() *Tracker {
 		ipv6UpdateRate:      metrics.NewMeter(),
 		announcementCounter: metrics.NewCounter(),
 		withdrawalCounter:   metrics.NewCounter(),
-		churnRate:           metrics.NewMeter(),
+		churnPerMinute:      minuteCounter{now: time.Now},
 	}
 }
 
@@ -156,13 +156,13 @@ func (t *Tracker) RecordIPv6Update() {
 // RecordAnnouncement records a route announcement
 func (t *Tracker) RecordAnnouncement() {
 	t.announcementCounter.Inc(1)
-	t.churnRate.Mark(1)
+	t.churnPerMinute.add()
 }
 
 // RecordWithdrawal records a route withdrawal
 func (t *Tracker) RecordWithdrawal() {
 	t.withdrawalCounter.Inc(1)
-	t.churnRate.Mark(1)
+	t.churnPerMinute.add()
 }
 
 // SetBGPPeerCount updates the current BGP peer count
@@ -198,9 +198,10 @@ func (t *Tracker) GetWithdrawalCount() uint64 {
 	return uint64(count)
 }
 
-// GetChurnRate returns the route churn rate per second
-func (t *Tracker) GetChurnRate() float64 {
-	return t.churnRate.Rate1()
+// GetChurnPerMinute returns the number of prefixes announced or withdrawn in
+// the last full minute of the clock.
+func (t *Tracker) GetChurnPerMinute() int {
+	return t.churnPerMinute.lastFullMinute()
 }
 
 // GetRouteMetrics returns current route update metrics