Whitelist node exporter metrics

This filters metrics after Prometheus scraped the endpoint and before it ingests them. This acts as a whitelist that we'll need to update every time we change or add a dashboard.

I used the query

count by (__name__, job)({__name__=~"node_.+"})

To check from the Prometheus web UI whether the whitelist was being applied or not. Note that due to the stage where the metrics_relabel_configs is performed, by checking the /metrics endpoint directly you want see any change, as that data is yet to be scraped (I did it until I understood the stages).