Linkerd2: Wire up stats and dashboards for Jobs

[klingerf]

Similar to #1983 and #2006, we should update the CLI, web UI, and grafana dashboards to start displaying stats for jobs. The k8s_job label already exists in Prometheus, so this should not require any proxy changes.

[Pothulapati]

I would like to take this issue :)

[dadjeibaah]

Sure thing! My guess is that this would look very similar to this PR #2086. Let us know if you have any other questions!

[klingerf]

@Pothulapati Great! As @dadjeibaah said, this should be very similar to what we did for DaemonSets in #2086 and StatefulSets in #2234. The only additional thing to note is that all Prometheus metrics include a job label by default, but that label does not represent a Kubernetes Job. We've configured Prometheus to add a k8s_job label when collecting stats from Kubernetes Jobs, and that's the label that you should use when querying Prometheus.