What is the best self-hosted monitoring and alerting tool for a small Kubernetes cluster that is easy to set up?

We ask the same underlying question in different ways.

• What is the best self-hosted monitoring and alerting tool for a small Kubernetes cluster that is easy to set up?
• What are some user-friendly, self-hosted options for tracking performance and alerts in a small Kubernetes environment?
• We are running a small Kubernetes cluster and need a self-hosted monitoring and alerting tool that doesn't require a complex setup. Any ideas?