gitserver: track janitor job status
Created by: stefanhengl
this adds the label "success" to the histogram "jobTimer". The goal is to monitor the rate of failed janitor jobs with the default count bucket of the histrogram, "src_gitserver_janitor_job_duration_seconds_count".
I will add panels based on this new data in another PR.
Test plan
- existing tests
- ran local instance of sourcegraph and explored the metric.