in_node_exporter_metrics: systemd: treat states as boolean values.

[pwhelan]

Summary

Rewrite the node_exporter_metrics so that the state of systemd units is a boolean instead of incrementing it, ie:

node_systemd_unit_state{...name="keepalived.service",state="active"} 12
node_systemd_unit_state{...name="keepalived.service",state="inactive"} 257

This would instead be:

node_systemd_unit_state{...name="keepalived.service",state="active"} 0
node_systemd_unit_state{...name="keepalived.service",state="inactive"} 1

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Testing
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Backporting

• Backport to latest stable release.

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[edsiper]

@pwhelan what's the main difference with the implementation proposed on #9134 ?

[pwhelan]

Replaced by #9134.