Implement OpenTelemetry for Go service

[davideme]

Overview

Implement the OpenTelemetry integration for the Go (Chi + sqlc) service as specified in src/go/adr/008-opentelemetry.md.

Parent issue: #14

Tasks

• Add required Go modules (go.opentelemetry.io/otel, otel/sdk, otlptracegrpc, otlpmetricgrpc, otelhttp, contrib/instrumentation/runtime, semconv/v1.26.0)
• Initialize SDK at startup — use no-op providers when OTEL_EXPORTER_OTLP_ENDPOINT is absent
• Wrap Chi router with otelhttp.NewHandler for inbound HTTP traces and metrics
• Wrap outbound HTTP clients with otelhttp.NewTransport
• Add manual span wrapping around sqlc repository calls (no auto-instrumentation available)
• Enable Go runtime metrics via go.opentelemetry.io/contrib/instrumentation/runtime (goroutines, heap, GC)
• Inject trace/span IDs into log/slog structured log entries
• Verify metrics baseline: http.server.request.duration, http.server.active_requests, go.goroutine.count, go.memory.used, go.gc.duration
• Set resource attributes: service.name=lamp-control-api-go, service.version, deployment.environment
• Ensure W3C TraceContext + Baggage propagation is enabled
• Add/update tests to verify instrumentation wires up correctly

Instrumentation summary

Signal	Mechanism	Type
Traces – Inbound HTTP	otelhttp.NewHandler	Config only
Traces – Outbound HTTP	otelhttp.NewTransport	Config only
Traces – Database (sqlc)	Manual span wrapping via OTel Tracer API	Custom code
Metrics – HTTP server	otelhttp (auto-emitted)	Config only
Metrics – Go runtime	contrib/instrumentation/runtime	Config only
Logs	Manual trace.SpanFromContext + log/slog injection	Custom code

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 ADR mandates migration from log to log/slog, which is correct. However, the implementation task should be more explicit about structured JSON output:

• Use slog.NewJSONHandler: Configure the default logger with slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{Level: logLevel}) so all log output is JSON-structured.
• Trace context utility: Create a helper function (e.g., logWithTrace(ctx context.Context, msg string, args ...any)) that extracts trace_id and span_id from trace.SpanFromContext(ctx) and adds them as structured slog attributes.
• Replace all log.Printf/log.Println calls in cmd/lamp-control-api/main.go and repository methods with slog equivalents.
• Log level via env: Support LOG_LEVEL environment variable to configure slog level.

Acceptance criterion to add: All log output is JSON (using slog.NewJSONHandler) and includes trace_id/span_id fields when a span is active.