feat(observability): OpenTelemetry metrics (Prometheus) + tracing (OTLP)

[kashkoool]

What

Production observability via OpenTelemetry.

• Metrics: ASP.NET Core + HttpClient + .NET runtime instrumentation plus custom business counters (payments succeeded/failed, bookings confirmed) over a Meter, exposed at GET /metrics for Prometheus. Gated by Observability:Metrics:Enabled (default on) and intended to be reached only from the internal network — firewall/proxy it (documented).
• Tracing: ASP.NET Core + HttpClient spans, exported via OTLP only when Observability:Otlp:Endpoint is configured (no noisy export failures when unset).
• IAppMetrics (Application abstraction) → AppMetrics (API Meter) wired into the payment webhook handler so payment/booking outcomes are recorded.
• Packages pinned in Directory.Packages.props (OpenTelemetry 1.15.x). Config in appsettings.json + .env.example. Serilog request logging stays.

Tests

ObservabilityTests: /metrics serves Prometheus text (OpenTelemetry boots OK). 53 unit + 33 integration green; 0 warnings.

Final phase of the roadmap (indexes ✅ → Google OAuth ✅ → staff ✅ → payments ✅ → notifications/real-time ✅ → reports/demand ✅ → observability). Cloud hosting/IaC remains deferred by decision.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 9d94cc2b-8b79-44a4-960e-1c0874864797

📥 Commits

Reviewing files that changed from the base of the PR and between 274ddae and 54fea98.

📒 Files selected for processing (10)

• .env.example
• Directory.Packages.props
• src/TransportPlatform.Api/Observability/AppMetrics.cs
• src/TransportPlatform.Api/Observability/ObservabilityExtensions.cs
• src/TransportPlatform.Api/Program.cs
• src/TransportPlatform.Api/TransportPlatform.Api.csproj
• src/TransportPlatform.Api/appsettings.json
• src/TransportPlatform.Application/Abstractions/IAppMetrics.cs
• src/TransportPlatform.Application/Payments/ProcessPaymentWebhook.cs
• tests/TransportPlatform.IntegrationTests/ObservabilityTests.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/observability

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