finer grain control of systemd metrics

As raised at https://github.com/prometheus/node_exporter/issues/1201#issuecomment-540264723, it isn't possible to get --collector.systemd.enable-restarts-metrics without pulling in other, very numerous metrics that come with --collector.systemd.

For example, on our prometheus deployment, metric node_systemd_unit_state represents 14% of our total metric key space. It's wasteful, as we only need node_systemd_service_restart_total (which is 2% of key space).

non-solution: "filter at the server side". We want to prevent these sizable, unneeded metrics from affecting the leaves and intermediaries of the system.

questionable-solution: "use systemd exporter". Why must we learn, configure, and deploy another exporter when node_exporter already has the metric we need?

Proposal: add a new flag to disable the default metrics of --collector.systemd (method collectUnitStatusMetrics of the collector)

https://github.com/prometheus/node_exporter/blob/8c3de12c2213c775217290d2b97f465c23f7bff6/collector/systemd_linux.go#L179

cc: @pgier

Start in collector/systemd_linux.go at collectUnitStatusMetrics and trace how --collector.systemd.enable-restarts-metrics is registered and used. Add the proposed control for the default systemd metrics, then verify that restart metrics remain available without the unwanted unit-state metrics. Done means the collector can emit only the requested restart metrics.