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
```

namespaces.go

@@ -191,6 +191,7 @@ func (h *Headscale) checkForNamespacesPendingUpdates() {
 			continue
 		}
 		for _, m := range *machines {
+			updateRequestsFromNode.WithLabelValues("namespace-update").Inc()
 			h.notifyChangesToPeers(&m)
 		}
 	}