Add anonymous usage metrics collection (#508)

This will help us understand usage on self-hosted deployments

The collection may be disabled by using the flag --disable-anonymous-metrics or 
NETBIRD_DISABLE_ANONYMOUS_METRICS in setup.env

management/server/updatechannel.go

@@ -70,3 +70,14 @@ func (p *PeersUpdateManager) CloseChannel(peerKey string) {
 
 	log.Debugf("closed updates channel of a peer %s", peerKey)
 }
+
+// GetAllConnectedPeers returns a copy of the connected peers map
+func (p *PeersUpdateManager) GetAllConnectedPeers() map[string]struct{} {
+	p.channelsMux.Lock()
+	defer p.channelsMux.Unlock()
+	m := make(map[string]struct{})
+	for key := range p.peerChannels {
+		m[key] = struct{}{}
+	}
+	return m
+}