Tag metrics with region and availability zone
Tag metrics with region and availability zone

This might be useful when diagnosing any future issues. It shouldn't
cost us anything extra series cardinality-wise because we're already
tagging with the host.

```
$ curl http://169.254.169.254/2016-09-02/meta-data/placement/availability-zone
eu-west-1a
```

openregister/openregister-java,openregister/openregister-java,openregister/openregister-java,openregister/openregister-java,openregister/openregister-java