worker: Add job metrics
Created by: efritz
This PR adds the following panels to the worker dashboard:
- An (alert-less, informational) panel with the count of worker instances processing each job type (e.g. codeintel-janitor, codeintel-commitgraph, etc)
- A panel for each job type with an alert that triggers when there are no instances processing that job. This should alert a site-manager to check their deployment configuration to ensure that there is at least one worker running each job.
Fixes https://github.com/sourcegraph/sourcegraph/issues/21760.
I'm doing metrics prior to docs so that we use screenshots of the dashboard when explaining how to configure the service.