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
```
This commit removes most of the locks in the PollingMap handler as there
was combinations that caused deadlocks. Instead of doing a plain map and
doing the locking ourselves, we use sync.Map which handles it for us.
tests for that feature.
Other fixes: clean up a few typos in comments. Fix a bug that caused the
tests to run four times each. Be more consistent in the use of log
rather than fmt to print errors and notices.
