[ Feature request ] Add usefull metrics

[hugosxm]

Is your feature request related to a problem? Please describe.
Hi ! We started to use your exporter for our pbs and find it very usefull, we would like to see more metrics, sorry I do not understand a thing at GO, i would have done a pull request...

Describe the solution you'd like
Here is what we would like to see :

• The garbage collection status with gc-status here
• There also some interresting metrics about GC here but the status is the more simple and usefull to write I guess
• The garbage collection last run time
• Sync status when you have several pbs should be possible with that, I can provide a test infrastructure and test the api calls for you if interested !

Describe any alternatives you've considered
None

Additional context
We tested with pbs3, its works flawlessly !