Implement OpenTelemetry for C# service

[davideme]

Overview

Implement the OpenTelemetry integration for the C# (ASP.NET Core + EF Core) service as specified in src/csharp/adr/008-opentelemetry.md.

Parent issue: #14

Tasks

• Add required NuGet packages (OpenTelemetry.Extensions.Hosting, OpenTelemetry.Instrumentation.AspNetCore, OpenTelemetry.Instrumentation.Http, OpenTelemetry.Instrumentation.EntityFrameworkCore, OpenTelemetry.Instrumentation.Runtime, OpenTelemetry.Exporter.OpenTelemetryProtocol)
• Register OTel services in Program.cs via AddOpenTelemetry() — OTLP exporters MUST NOT be registered when OTEL_EXPORTER_OTLP_ENDPOINT is absent
• Configure AddAspNetCoreInstrumentation for inbound HTTP traces and metrics
• Configure AddHttpClientInstrumentation for outbound HTTP traces
• Configure AddEntityFrameworkCoreInstrumentation for database traces
• Configure AddRuntimeInstrumentation for .NET runtime metrics (GC, threads, heap)
• Bridge ILogger to OTel via AddOpenTelemetry() log provider
• Verify metrics baseline: http.server.request.duration, http.server.active_requests, .NET runtime metrics
• Set resource attributes: service.name=lamp-control-api-csharp, service.version, deployment.environment
• Support both grpc and http/protobuf protocols via OTEL_EXPORTER_OTLP_PROTOCOL
• Ensure W3C TraceContext + Baggage propagation is enabled
• Add/update tests to verify instrumentation wires up correctly

Instrumentation summary

Signal	Mechanism	Type
Traces – Inbound HTTP	AddAspNetCoreInstrumentation	Config only
Traces – Outbound HTTP	AddHttpClientInstrumentation	Config only
Traces – Database (EF Core)	AddEntityFrameworkCoreInstrumentation	Config only
Metrics – HTTP server	AddAspNetCoreInstrumentation	Config only
Metrics – .NET runtime	AddRuntimeInstrumentation	Config only
Logs	OTel ILogger bridge	Config only

Acceptance criteria

• OTLP exporters are not registered 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 ILogger-to-OTel bridge (builder.Logging.AddOpenTelemetry()) exports logs to the OTel collector, but the console output format is not addressed. ASP.NET Core's default ILogger console format is plain text.

The following should be added to the implementation tasks:

• Structured JSON console output: Add builder.Logging.AddJsonConsole() in Program.cs (or configure via appsettings.json: "Console": { "FormatterName": "json" }) for structured log output.
• Trace context in logs: The OTel .NET SDK automatically injects TraceId and SpanId into the ILogger scope when an active span exists — these will appear as structured fields in JSON console output.
• Conditional configuration: JSON console logging should be active regardless of whether OTEL_EXPORTER_OTLP_ENDPOINT is set (structured logs are useful even without a collector).

Acceptance criterion to add: Console log output is structured JSON, with TraceId and SpanId fields present when a span is active.