Initial work on Prometheus metrics

This commit adds some Prometheus metrics to /metrics in headscale.

It will add the standard go metrics, some automatic gin metrics and some
initial headscale specific ones.

Some of them has been added to aid debugging #97 (loop bug)

In the future, we can use the metrics to get rid of the sleep in the
integration tests by checking that our expected number of nodes has been
registered:

```
headscale_machine_registrations_total
```

metrics.go

@@ -0,0 +1,41 @@
+package headscale
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+const prometheusNamespace = "headscale"
+
+var (
+	// This is a high cardinality metric (namespace x machines), we might want to make this
+	// configurable/opt-in in the future.
+	lastStateUpdate = promauto.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: prometheusNamespace,
+		Name:      "last_update_seconds",
+		Help:      "Time stamp in unix time when a machine or headscale was updated",
+	}, []string{"namespace", "machine"})
+
+	machineRegistrations = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: prometheusNamespace,
+		Name:      "machine_registrations_total",
+		Help:      "The total amount of registered machine attempts",
+	}, []string{"action", "auth", "status", "namespace"})
+
+	updateRequestsFromNode = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: prometheusNamespace,
+		Name:      "update_request_from_node_total",
+		Help:      "The number of updates requested by a node/update function",
+	}, []string{"state"})
+	updateRequestsToNode = promauto.NewCounter(prometheus.CounterOpts{
+		Namespace: prometheusNamespace,
+		Name:      "update_request_to_node_total",
+		Help:      "The number of calls/messages issued on a specific nodes update channel",
+	})
+	//TODO(kradalby): This is very debugging, we might want to remove it.
+	updateRequestsReceivedOnChannel = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: prometheusNamespace,
+		Name:      "update_request_received_on_channel_total",
+		Help:      "The number of update requests received on an update channel",
+	}, []string{"machine"})
+)