The OpenTelemetry Spring Boot starter (v2.26.0+) now supports declarative configuration via a YAML schema embedded directly in application.yaml under an otel: key. This post traces how a single environment variable (OTEL_SERVICE_NAME) flows through Spring's property stack, gets normalized, and reaches the SDK. It explains the three stages: Spring's property unification, the bracket-index naming gap the starter bridges manually, and the difference between Spring's ${VAR:default} placeholder resolution and the SDK's own substituter. Practical benefits include replacing @Configuration beans with YAML blocks (e.g., excluding actuator endpoints from tracing), automatic env var overrides without pre-wiring placeholders, and compatibility with Spring profiles and external config servers. The post also covers migration gotchas like Spring Boot 3.5 BOM conflicts and the new DeclarativeConfigurationCustomizerProvider API.
