Add more metrics (#450)

* Add more metrics

This change adds the following counter metrics:

Metrics to track packets dropped at the firewall:

    firewall.dropped.local_ip
    firewall.dropped.remote_ip
    firewall.dropped.no_rule

Metrics to track handshakes attempts that have been initiated and ones
that have timed out (ones that have completed are tracked by the
existing "handshakes" histogram).

    handshake_manager.initiated
    handshake_manager.timed_out

Metrics to track when cached_packets are dropped because we run out of
buffer space, and how many are sent once the handshake completes.

    hostinfo.cached_packets.dropped
    hostinfo.cached_packets.sent

This change also notes how many cached packets we have when we log the
final "Handshake received" message for either stage1 for stage2.

* separate incoming/outgoing metrics

* remove "allowed" firewall metrics

We don't need this on the hotpath, they aren't worh it.

* don't need pointers here

interface.go

@@ -77,9 +77,11 @@ type Interface struct {
 	writers []*udpConn
 	readers []io.ReadWriteCloser
 
-	metricHandshakes metrics.Histogram
-	messageMetrics   *MessageMetrics
-	l                *logrus.Logger
+	metricHandshakes    metrics.Histogram
+	messageMetrics      *MessageMetrics
+	cachedPacketMetrics *cachedPacketMetrics
+
+	l *logrus.Logger
 }
 
 func NewInterface(c *InterfaceConfig) (*Interface, error) {
@@ -122,7 +124,12 @@ func NewInterface(c *InterfaceConfig) (*Interface, error) {
 
 		metricHandshakes: metrics.GetOrRegisterHistogram("handshakes", nil, metrics.NewExpDecaySample(1028, 0.015)),
 		messageMetrics:   c.MessageMetrics,
-		l:                c.l,
+		cachedPacketMetrics: &cachedPacketMetrics{
+			sent:    metrics.GetOrRegisterCounter("hostinfo.cached_packets.sent", nil),
+			dropped: metrics.GetOrRegisterCounter("hostinfo.cached_packets.dropped", nil),
+		},
+
+		l: c.l,
 	}
 
 	ifce.connectionManager = newConnectionManager(c.l, ifce, c.checkInterval, c.pendingDeletionInterval)