Observability means you can answer “what is wrong and why” from your system’s outputs alone — without adding new instrumentation after an incident. It requires three types of data: metrics (what happened), traces (why it happened), and logs (the details). Spring Boot 4 ships a single OpenTelemetry starter that covers all three. This guide shows how to wire up the complete observability stack: Prometheus + Grafana for metrics, Grafana Tempo for distributed tracing, and Grafana Loki for logs.