Implement OpenTelemetry for Java service

[davideme]

Overview

Implement the OpenTelemetry integration for the Java (Spring Boot + JPA) service as specified in src/java/adr/010-opentelemetry.md.

Parent issue: #14

Tasks

• Add OTel Java Agent JAR to the build (download via Maven or bundle in repo)
• Configure JVM startup flag: -javaagent:opentelemetry-javaagent.jar
• Add opentelemetry-logback-appender-1.0 Maven dependency and configure Logback appender
• Add opentelemetry-runtime-telemetry-java17 for JFR-based JVM runtime metrics
• Configure Micrometer OTel bridge for Spring Boot Actuator metrics export
• Update application.yml / env vars for OTEL_SERVICE_NAME=lamp-control-api-java, OTEL_EXPORTER_OTLP_ENDPOINT, OTEL_LOGS_EXPORTER=otlp
• Verify zero-code instrumentation covers: Spring MVC (HTTP), RestTemplate/WebClient (outbound), Hibernate/JDBC (database)
• Verify metrics baseline: http.server.request.duration, http.server.active_requests, JVM runtime metrics
• Set resource attributes: service.name, service.version, deployment.environment
• Ensure W3C TraceContext + Baggage propagation is enabled (default in agent)
• Add/update tests to verify instrumentation wires up correctly

Instrumentation summary

Signal	Mechanism	Type
Traces – Inbound HTTP	OTel Java Agent (Spring MVC)	Zero-code
Traces – Outbound HTTP	OTel Java Agent (RestTemplate/WebClient)	Zero-code
Traces – Database	OTel Java Agent (Hibernate/JDBC)	Zero-code
Metrics – HTTP server	OTel Java Agent + Micrometer bridge	Zero-code
Metrics – JVM runtime	opentelemetry-runtime-telemetry-java17	Zero-code
Logs	opentelemetry-logback-appender-1.0	Config only

Acceptance criteria

• Agent is a no-op when OTEL_EXPORTER_OTLP_ENDPOINT is not set
• All three pillars (traces, metrics, logs) emit data when collector is configured
• Existing tests continue to pass

[davideme]

Structured Logging Requirement

The opentelemetry-logback-appender-1.0 bridges Logback logs to the OTel log signal (OTLP export), but this does not change the console log format to structured JSON. A separate structured logging configuration is needed:

• Add JSON encoder for Logback: Add logstash-logback-encoder to pom.xml (or use Spring Boot 3.4+'s built-in logging.structured.format.console=ecs in application.properties).
• Configure logback-spring.xml: Create/update with a JSON appender pattern so console output is structured JSON in production (can use Spring profiles to keep human-readable format in development).
• OTel MDC fields: When the OTel Logback appender is active and a span is in scope, trace_id and span_id are available as MDC fields — ensure the JSON encoder includes MDC in the log output.

Without this, logs emitted to stdout are plain text even when the OTel Logback appender is configured.

Acceptance criterion to add: Console log output is structured JSON in production mode, with trace_id and span_id fields present when a span is active.