Work with upstream to make node_exporter integration easier to maintain

[rfratto]

grafana/agent#1228 updated our dependency on node_exporter from v1.0.1 to v1.3.1. This ended up being a lot more work than I thought it would be, primarily due to its mapping of YAML to the global kingpin flags node_exporter creates.

It will be easier (and less error prone) to maintain the node_exporter integration in the future if we work with upstream to make the integration easier to maintain. I haven't put a ton of thought into it, but I think this at least removing the usage of unexported globals that configure collectors.

[marctc]

WIP PR: prometheus/node_exporter#2812