The debate between self-hosted open-source observability (LGTM stack: Loki, Grafana, Tempo, Mimir / Prometheus) and unified SaaS platforms like Datadog is not merely a software license debate—it is an engineering tradeoff between infrastructure management overhead, operational headcount, high-cardinality data scale, and unpredictable SaaS billing traps.
Key Architectural Takeaway: Datadog delivers turnkey, zero-maintenance APM, log, and metric correlation with zero operational headcount, but penalizes high-cardinality metrics with aggressive SaaS pricing. Prometheus, Grafana, and OpenTelemetry eliminate SaaS vendor markups and prevent lock-in, but demand dedicated SRE infrastructure maintenance.