exporter fails if job_stats not enabled

[sknolin]

This exporter does not work if jobstats are not enabled.

The metrics result is

An error has occurred during metrics gathering:

64 error(s) occurred:
* collected metric lustre_job_read_samples_total label:<name:"component" value:"ost" > label:<name:"jobid" value:".0" > label:<name:"target" value:"scratch-OST0008" > counter:<value:0 >  was collected before with the same name and label values ... (etc)

To cope, I simply made all jobstats features 'extended' for now.

[sknolin]

changes to make jobstats extended:
https://github.com/sknolin/lustre_exporter/commit/3ba07b37ce5cd77c526b7eea7dd6bcbbaf7984b7

[sknolin]

I see this is a duplicate of #135

version 2.10.6-1

So I assume with lustre disabled it works ok but returns no data. Even so, we sometimes want jobstats on but not to collect with prometheus.