Implement OpenTelemetry for Kotlin service

[davideme]

Overview

Implement the OpenTelemetry integration for the Kotlin (Ktor + Exposed) service as specified in src/kotlin/adr/007-opentelemetry.md.

Parent issue: #14

Tasks

• Add required Gradle dependencies (opentelemetry-ktor-3.0, opentelemetry-sdk, opentelemetry-exporter-otlp, opentelemetry-runtime-telemetry-java8, opentelemetry-logback-appender-1.0)
• Initialize OTel SDK at app startup — use no-op when OTEL_EXPORTER_OTLP_ENDPOINT is absent
• Install KtorServerTelemetry plugin for inbound HTTP traces and metrics
• Install KtorClientTelemetry plugin for outbound HTTP traces
• Add manual span wrapping around Exposed ORM calls (no auto-instrumentation available)
• Configure Logback appender (opentelemetry-logback-appender-1.0) for log → OTel bridge
• Enable JVM runtime metrics via opentelemetry-runtime-telemetry-java8
• Verify metrics baseline: http.server.request.duration, http.server.active_requests, JVM runtime metrics
• Set resource attributes: service.name=lamp-control-api-kotlin, service.version, deployment.environment
• Configure OTEL_TRACES_EXPORTER=none, OTEL_METRICS_EXPORTER=none, OTEL_LOGS_EXPORTER=none as safe defaults (set to otlp when endpoint configured)
• Ensure W3C TraceContext + Baggage propagation is enabled
• Add/update tests to verify instrumentation wires up correctly

Instrumentation summary

Signal	Mechanism	Type
Traces – Inbound HTTP	KtorServerTelemetry plugin	Config only
Traces – Outbound HTTP	KtorClientTelemetry plugin	Config only
Traces – Database (Exposed)	Manual span wrapping via OTel Tracer API	Custom code
Metrics – HTTP server	KtorServerTelemetry (auto-emitted)	Config only
Metrics – JVM runtime	opentelemetry-runtime-telemetry-java8	Config only
Logs	opentelemetry-logback-appender-1.0	Config only

Acceptance criteria

• SDK 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 current logback.xml uses a plain text pattern:

<pattern>%d{YYYY-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>

The opentelemetry-logback-appender-1.0 bridges logs to OTel, but the console output remains plain text. The following should be added:

• Add JSON encoder: Add logstash-logback-encoder to build.gradle.kts and update logback.xml to use net.logstash.logback.encoder.LogstashEncoder for the console appender.
• MDC trace fields: When the OTel Logback appender is active and a span is in scope, trace_id and span_id are available as MDC fields — LogstashEncoder includes MDC automatically.
• Profile-aware config: Consider keeping a human-readable pattern for local development (e.g., via Ktor environment detection) and JSON for production.

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.