MAUI OTel: force TracerProvider materialization since IHostedService doesn't run

[davidortinau]

Summary

MAUI's MauiApp implements IHostApplicationBuilder but does not run IHostedService instances (no IHost.StartAsync loop). OpenTelemetry.Extensions.Hosting registers a TelemetryHostedService whose StartAsync is what materializes the TracerProvider, attaches its ActivityListeners, and begins exporting. On MAUI, that service never runs — so:

• Logs still work (they hook ILoggerFactory synchronously during AddOpenTelemetry()).
• Auto-instrumentation (AddHttpClientInstrumentation) does not — no listener → no spans → no traceparent header injection → API sees every request as a root span and correlation joins return zero rows in Application Insights.

Evidence from this session

• OpenTelemetry.Instrumentation.Http 1.15.0 is present, .AddHttpClientInstrumentation() has been on the TracerProvider since commit 216a2da1 (March 4).
• Ran a trim-disabled (<MtouchLink>None</MtouchLink>) Release build on DX24; bundle 136 MB / 333 DLLs — trimming disproven.
• KQL showed 127 mobile traces with empty operation_Id on every entry → no ambient Activity ever existed on the device.
• Existing OpenTelemetryInitializer : IMauiInitializeService used nullable services.GetService<TracerProvider>() — may have silently returned null.

Workaround shipped

PR #XXX (replace) adds an ApiActivityHandler DelegatingHandler and a dedicated ActivitySource (SentenceStudio.Mobile.HttpClient) explicitly registered with the TracerProvider. It also hardens OpenTelemetryInitializer to use GetRequiredService<TracerProvider>() to force materialization. This makes correlation work today but does not address the root cause.

Root-cause fix (this issue)

Decide between:

1. Explicitly call TelemetryHostedService.StartAsync from OpenTelemetryInitializer — get every auto-instrumentation listener attached for free, no per-call handler needed.
2. Build TracerProvider directly via Sdk.CreateTracerProviderBuilder() instead of going through services.AddOpenTelemetry().WithTracing(...), bypassing the hosted-service dependency entirely.
3. Upstream fix — ask the OpenTelemetry maintainers for a non-IHostedService bootstrap path for IHostApplicationBuilder implementations that don't run hosted services (MAUI, some WPF/WinForms IHost scenarios).

If option 1 or 2 lands, the ApiActivityHandler can stay (it's still a correct Client span for each API call) and the AddHttpClientInstrumentation() spans become additive. No code needs to be reverted.

Acceptance criteria

• On a clean install of the mobile app, a single API call produces:
  • At least one dependencies row in App Insights from the mobile side.
  • A matching requests row on the server with equal operation_Id to that dependency.
  • Server operation_Id != operation_ParentId (i.e., mobile traceparent actually propagated).
• Works without the manual ApiActivityHandler being the only source.

Links

• PR Mobile App Insights: small slice (Mac Catalyst) #165 — mobile App Insights bootstrap
• PR Server-side App Insights: close mobile↔API correlation loop #166 — server companion
• Session decision: .squad/decisions/inbox/wash-server-appinsights-shipped.md (merged)

[davidortinau]

Update: the user-space workaround shipped in PR #172 (manual ApiActivityHandler DelegatingHandler + GetRequiredService<T>() hardening) and PR #173 (explicit DistributedContextPropagator.Current.Inject before base.SendAsync) is now verified working end-to-end on production DX24.

KQL confirmation (AppId 74e94530-d17f-404a-8726-b7266724b70f, 15m window):

• Mobile dependencies: 39 rows with populated operation_Id
• Mobile→API correlation join: 20 joined rows across /api/v1/ai/chat and full /api/sync-agent/* flow, all 200 status

This downgrades the priority of the underlying framework fix. The root cause — MAUI's MauiApp doesn't run IHostedService, so OTel's TelemetryHostedService.StartAsync never materializes the TracerProvider and attaches listeners to System.Net.Http — still stands and is worth addressing upstream (either by MAUI running HostedServices, or by OTel's AddHttpClientInstrumentation eagerly building a TracerProvider / attaching listeners without hosted-service startup). But there's no production pain now; keeping this open for framework-level tracking only.