Drop node-exporter systemd metrics for units we do not care about

Some of these systemd metrics have large cardinalities, so it's good
to prune them at collection time.

Part of optimization work for issue #51.
1 job for master in 5 minutes and 9 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #8905
ai3
run_base_test

00:05:09