[Metricbeat] Support systemd version from RHEL 7 for system service metricset

[hochraldo]

RHEL 7 uses systemd version 219. Therefore the system service metricset does not work as the ListUnitsByPatterns (see

beats/metricbeat/module/system/service/service.go

Line 80 in d89675f

units, err := m.conn.ListUnitsByPatterns(m.cfg.StateFilter, []string{"*.service"})

) method call did not exist in that version.

It seems that this method call was introduced in a later version of systemd: systemd/systemd@313fe66

As an alternative ListUnitsFiltered could be used, as this method call is available since earlier versions of systemd and provides similar results to my understanding.

RHEL 7 is a widely used Linux system in enterprises which will get "normal" support until 30 June 2024.

[jsoriano]

Hey @hochraldo, thanks for reporting, I agree that it would be nice to support RHEL 7.

cc @fearful-symmetry

[fearful-symmetry]

I'll see what I can do. I agree, we should support this. Might require some hacking depending on what our vendored library supports.

[nabeel-shakeel]

@fearful-symmetry: service metricset not generating any event on CentOS after this fix.