codeintel: Increase background task observability

Created by: efritz

This adds more observability around the background operations for code intel that run in the frontend. These include a commit updater, the performance and opaqueness of which caused recent customer upgrade pains, as well as a set of janitor processes that keep multiple data stores consistent.

We now have greater tracing observability around the commit updater with more detailed trace logs to better show the scale of data being processed within execution traces. We also updated the monitoring dashboard with:

• commit updater errors and duration
• dirty repository queue growth rate
• number of records removed by a janitor process

Closes https://github.com/sourcegraph/sourcegraph/issues/16527.