Devise a metric for ci_builds table state/health

Overview

Before we begin making changes to the ci_builds table we should establish baseline metrics for the relative table health and state. We should have these metrics in place so that we do not take on unnecessary risk, and to ensure that our changes are affecting the state and health of ci_builds in a positive way.

Proposal

It is not clear what would be the best metric, but the most simple one might be the table size and total indices size.

A grafana dashboard exists tracking the used keyspace of the ci_builds table here.