When using Kubernetes on cloud providers, nodes will have the
spec.providerID field populated to contain the cloud provider specific
name of the EC2/GCE/... instance.
Let's expose this information as an additional label, so that it's
easier to annotate metrics and alerts to contain the cloud provider
specific name of the instance to which it pertains.
Signed-off-by: Ed Schouten <eschouten@apple.com>