.NET: [Feature]: .NET - Allow enabling workflow-level OpenTelemetry when using HandoffWorkflowBuilder

@TaoChenOSU is already working on this.

Since Jun 4, 2026.

### Description

When building a handoff workflow with `HandoffWorkflowBuilder`, there does not appear to be a public way to enable workflow-level OpenTelemetry instrumentation.

The lower-level `WorkflowBuilder` already supports workflow telemetry through `WithOpenTelemetry(...)`, which is useful for observing workflow execution spans such as:

* `workflow.build`
* `workflow.session` / `workflow_invoke`
* `edge_group.process`
* `executor.process`
* `message.send`

However, `HandoffWorkflowBuilder` encapsulates the internal `WorkflowBuilder` instance. In `Build()`, it creates an internal `WorkflowBuilder`, wires the handoff start/end executors and `HandoffAgentExecutor` instances, then directly builds the workflow. Because the internal builder is not exposed and no telemetry-specific method is available on `HandoffWorkflowBuilder`, users can enable telemetry at the agent and/or chat client layers, but cannot easily enable the workflow-level telemetry that explains the handoff routing itself.

This is especially important for handoff workflows because the interesting production debugging signals are often at the workflow/routing layer, not only at the LLM call layer:

* which agent owned the current turn
* whether a handoff actually occurred
* which agent handed off to which target agent
* whether the message was delivered, dropped, buffered, or failed during edge processing
* where a run stopped: workflow, executor, edge routing, message delivery, approval, checkpoint, or model/tool call

### Code Sample

```markdown
### Current behavior

This style is supported when using `WorkflowBuilder` directly:

Workflow workflow = new WorkflowBuilder(startExecutor)
.AddEdge(startExecutor, nextExecutor)
.WithOutputFrom(nextExecutor)
.WithOpenTelemetry(options =>
{
options.EnableSensitiveData = false;
}, activitySource)
.Build();

But this style does not appear to be available for handoff workflows:

Workflow workflow = AgentWorkflowBuilder
.CreateHandoffBuilderWith(triageAgent)
.WithHandoff(triageAgent, refundAgent, "Handle refund requests.")
.WithHandoff(refundAgent, triageAgent, "Return to triage when the refund flow is complete.")
// No apparent way to call WithOpenTelemetry(...) for the internal WorkflowBuilder.
.Build();

### Expected behavior

It should be possible to opt into workflow-level OpenTelemetry when using `HandoffWorkflowBuilder`, without exposing or mutating the internal workflow graph.

For example:

Workflow workflow = AgentWorkflowBuilder
.CreateHandoffBuilderWith(triageAgent)
.WithHandoff(triageAgent, refundAgent, "Handle refund requests.")
.WithHandoff(refundAgent, triageAgent, "Return to triage when the refund flow is complete.")
.WithOpenTelemetry(options =>
{
options.EnableSensitiveData = false;
}, activitySource)
.Build();

This would allow users to observe workflow-level spans for the handoff orchestration while keeping agent-level and chat-client-level telemetry separately configurable.

### Proposed API

Add a telemetry pass-through method to `HandoffWorkflowBuilderCore`:

public TBuilder WithOpenTelemetry(
Action? configure = null,
ActivitySource? activitySource = null)
{
// Store the configuration and apply it to the internal WorkflowBuilder before Build().
}

Then apply it inside `Build()` before the internal workflow is built:

public Workflow Build()
{
HandoffsStartExecutor start = new(this._returnToPrevious);
HandoffsEndExecutor end = new(this._returnToPrevious);
WorkflowBuilder builder = new(start);

if (this._workflowTelemetryEnabled)
{
builder.WithOpenTelemetry(this._configureWorkflowTelemetry, this._activitySource);
}

// Existing handoff graph construction remains unchanged.

return builder.WithOutputFrom(end).Build();
}
```

### Language/SDK

.NET