Bridge Azure Durable Functions host OTel activities to Datadog spans

[chemystery09]

Summary

• Adds a DurableTaskActivityHandler that subscribes to the WebJobs.Extensions.DurableTask ActivitySource emitted by the Azure Functions host when Distributed Tracing V2 is enabled in host.json.
• Maps host-side durable task spans (orchestration lifecycle, create-orchestration, entity, timer, etc.) into Datadog with span.type=serverless and durabletask.* tags preserved.
• Complements worker-side trace context propagation in dd-trace-js#9394, which links activity/entity invocations to the HTTP root but intentionally does not emit orchestration spans from the worker (replay-unsafe).

Motivation

Orchestration spans are emitted by the .NET Functions host (Durable extension), not by language workers. Application Insights already receives these spans when V2 tracing is enabled; this change bridges the same host Activity events into Datadog via the existing OTel ActivityListener infrastructure (QuartzActivityHandler, AzureServiceBusActivityHandler, etc.).

Related internal tracking: SVLS-7644 / APMSVLS-332.

Requirements

1. Host config — enable V2 distributed tracing in host.json:

   {
     "extensions": {
       "durableTask": {
         "tracing": {
           "distributedTracingEnabled": true,
           "version": "V2"
         }
       }
     }
   }

2. Tracer config — OTel activity listener enabled (DD_TRACE_OTEL_ENABLED=true or equivalent).
3. Worker — dd-trace worker instrumentation (e.g. dd-trace-js #9394) for activity/entity server spans linked via propagated traceContext.

Implementation notes

• Handler registered in ActivityHandlersRegister before DefaultActivityHandler.
• Span resource names derived from durabletask.task.name + durabletask.type when present.
• Host activity client spans may appear alongside worker server spans for the same logical step; deduplication/filtering can be refined in a follow-up.

Test plan

• Unit tests for resource-name mapping, tag extraction, and ActivityKind → span kind mapping
• Integration test with Durable Functions sample app + V2 host.json (follow-up)
• Verify end-to-end trace: HTTP trigger → host orchestration span → worker activity span

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