observability: add kubernetes resource monitoring / unified set of container metrics across deployment types

Created by: slimsag

Currently, we have a Containers dashboard but it only works in the deploy-sourcegraph-docker context because it relies on the metrics only present in that context. Main problem:

• Kubernetes and deploy-sourcegraph-docker likely have the same set of metrics, but named slightly differently (we can use rules to rename them and define a common set).
• Single container deployments won't have any container metrics, so maybe it shouldn't have a Containers dashboard for now?