[management] Add metrics for peer status updates and ephemeral cleanup (#6196)

* [management] Add metrics for peer status updates and ephemeral cleanup

The session-fenced MarkPeerConnected / MarkPeerDisconnected path and
the ephemeral peer cleanup loop both run silently today: when fencing
rejects a stale stream, when a cleanup tick deletes peers, or when a
batch delete fails, we have no operational signal beyond log lines.

Add OpenTelemetry counters and a histogram so the same SLO-style
dashboards that already exist for the network-map controller can cover
peer connect/disconnect and ephemeral cleanup too.

All new attributes are bounded enums: operation in {connect,disconnect}
and outcome in {applied,stale,error,peer_not_found}. No account, peer,
or user ID is ever written as a metric label — total cardinality is
fixed at compile time (8 counter series, 2 histogram series, 4 unlabeled
ephemeral series).

Metric methods are nil-receiver safe so test composition that doesn't
wire telemetry (the bulk of the existing tests) works unchanged. The
ephemeral manager exposes a SetMetrics setter rather than taking the
collector through its constructor, keeping the constructor signature
stable across all test call sites.

* [management] Add OpenTelemetry metrics for ephemeral peer cleanup

Introduce counters for tracking ephemeral peer cleanup, including peers pending deletion, cleanup runs, successful deletions, and failed batches. Metrics are nil-receiver safe to ensure compatibility with test setups without telemetry.

management/internals/server/controllers.go

@@ -112,7 +112,11 @@ func (s *BaseServer) AuthManager() auth.Manager {
 
 func (s *BaseServer) EphemeralManager() ephemeral.Manager {
 	return Create(s, func() ephemeral.Manager {
-		return manager.NewEphemeralManager(s.Store(), s.PeersManager())
+		em := manager.NewEphemeralManager(s.Store(), s.PeersManager())
+		if metrics := s.Metrics(); metrics != nil {
+			em.SetMetrics(metrics.EphemeralPeersMetrics())
+		}
+		return em
 	})
 }