monitoring: Add panels to track table bloat

Created by: efritz

Fixes https://github.com/sourcegraph/sourcegraph/issues/18284. This adds panels to the Postgres dashboard to alert when groups of code intelligence tables which are known to get easily bloated in some deployments get disproportionately large.

This also adds a utility function to easily track the same thing for other groups of tables.

Created by: sourcegraph-bot

Notifying subscribers in CODENOTIFY files for diff 9295ae164c38becc9e9dbed50ce2e21a4eee37e4...57a1ba559e9c07904c2ef277f617d07c265b0480.

Notify	File(s)
@bobheadxi	monitoring/definitions/postgres.go
@christinaforney	doc/admin/observability/alert_solutions.md doc/admin/observability/dashboards.md
@slimsag	monitoring/definitions/postgres.go
@sourcegraph/distribution	doc/admin/observability/alert_solutions.md doc/admin/observability/dashboards.md monitoring/definitions/postgres.go