`prometheus_metrics` table should include non-null restrictions on all required columns
Summary
We often look to DB rules to best understand the assumptions we can make about the data we're interacting with. When tables and models are out of sync, understanding the data gets more difficult.
We should ensure the table corresponding to the PrometheusMetric
model has appropriate null: false
restrictions across all required columns. For additional context, see https://gitlab.com/gitlab-org/gitlab-ce/issues/66177#note_208118109.