add meta packet statistics (#230)

This change add more metrics around "meta" (non "message" type packets).
For lighthouse packets, we also record statistics around the specific
lighthouse meta type.

We don't keep statistics for the "message" type so that we don't slow
down the fast path (and you can just look at metrics on the tun
interface to find that information).

hostmap.go

@@ -30,6 +30,7 @@ type HostMap struct {
 	vpnCIDR         *net.IPNet
 	defaultRoute    uint32
 	unsafeRoutes    *CIDRTree
+	metricsEnabled  bool
 }
 
 type HostInfo struct {
@@ -384,8 +385,16 @@ func (hm *HostMap) PunchList() []*udpAddr {
 }
 
 func (hm *HostMap) Punchy(conn *udpConn) {
+	var metricsTxPunchy metrics.Counter
+	if hm.metricsEnabled {
+		metricsTxPunchy = metrics.GetOrRegisterCounter("messages.tx.punchy", nil)
+	} else {
+		metricsTxPunchy = metrics.NilCounter{}
+	}
+
 	for {
 		for _, addr := range hm.PunchList() {
+			metricsTxPunchy.Inc(1)
 			conn.WriteTo([]byte{1}, addr)
 		}
 		time.Sleep(time.Second * 30)