[thanos] - disable thanos ruler

What

Disables the thanos ruler.

Why

Our metric backend was recently migrated to Mimir. This means the rule processing is now also handled through Mimir.

With that completed we are now in the process of decommissioning Thanos. As a first step we are stopping existing metric ingestion and rule evaluations. This will disable the rule evaluations but keep the Thanos query components for historical data.

Related Issue: gitlab-com/gl-infra/scalability#3564 (closed)