Provide optional Cluster label for all metrics

Currently the provided grafana dashboard, using the prometheus exporter, is really only useful for one cluster. If you are running multiple clouds/clusters and want to see information for just one, then you need to heavily customize the dashboard.

We could have the ability for users to optionally provide a name for a cluster when starting the prometheus exporter, and that cluster name would be added as a label for all metrics produced by the prometheus exporter. Then we can have the idea of "clusters" built-in to the provided grafana dashboard, allowing users that run multiple clouds to use it much more easily.

In the dashboard, we can initially just have a "filter" for the cluster at the top.

We might think about giving the ability to add other custom labels to all metrics, but this would be a good start.