Push executor metrics

Created by: eseliger

Local dashboard:

Follow-up tasks:

• Implement push-through metrics for node_exporter on executors host
• Implement push-through metrics for node_exporter on registry host
• Expose from worker, not frontend
• Remove metrics setup from docs
• Clean up terraform modules
  • Remove credentials
  • Remove open port
  • Remove HTTP firewall rule
• Clean up k8s prometheus config
• Clean up Cloud prometheus config
• Clean up k8s secrets
• Clean up Cloud secrets
• Port this change to MI V1.0 and MI V1.1

Partner PRs

• https://github.com/sourcegraph/deploy-sourcegraph-docker/pull/836
• https://github.com/sourcegraph/deploy-sourcegraph-helm/pull/147
• https://github.com/sourcegraph/deploy-sourcegraph/pull/4149

Test plan

Validated locally that it still works.