[Metricbeat] System service allow to filter processes

[hochraldo]

Currently the processes to list can only be filtered by state (service.state_filter). In addition filtering by "module" name should be supported.

This module name pattern is hardcoded at the moment:
units, err := m.conn.ListUnitsByPatterns(m.cfg.StateFilter, []string{"*.service"})

This feature would allow to list services that are running under the same user/group and would not need "superuser" power.

[jsoriano]

cc @fearful-symmetry

[fearful-symmetry]

I'll see what I can do.