fix(telemetry): emit gen_ai.workflow.nested for nested workflows

The `gen_ai.workflow.nested=true` attribute/dimension was never produced for
any nested workflow in `gen_ai.invoke_workflow.duration` metric, so root and
nested workflow invocations could not be distinguished.

Root cause was a lost OpenTelemetry context value in
`_use_invoke_workflow_span`. The "entrypoint workflow active" marker was only
`set_value`'d onto the local `otel_context` handed to
`tracer.start_as_current_span(context=...)`. OTel uses that argument solely to
resolve the new span's parent; it does not attach it as the current context.
The child telemetry context was then captured from `context_api.get_current()`,
which never carried the marker, so every downstream workflow computed
`nested=False`.

Fix: after starting the span, `context_api.attach` the marker onto the current
context (and `detach` on exit) so child nodes capturing `get_current()` observe
`nested=True`. The now-redundant `set_value` onto the local context is removed.

Extend the telemetry functional tests so the scenario runs a workflow nested
inside a workflow. All expected span trees and the invoke_workflow duration
metric now assert the inner workflow carries `gen_ai.workflow.nested=true` while
the root omits it. This case previously went uncovered.

Co-authored-by: Max Ind <maxind@google.com>
PiperOrigin-RevId: 943153631

Author: RKest

src/google/adk/telemetry/node_tracing.py

@@ -186,10 +186,6 @@ def _use_invoke_workflow_span(
   # The flag rides along the otel_context propagated to child nodes, so nested
   # workflows see it set.
   nested = bool(context_api.get_value(_ENTRYPOINT_WORKFLOW_KEY, otel_context))
-  if not nested:
-    otel_context = context_api.set_value(
-        _ENTRYPOINT_WORKFLOW_KEY, True, otel_context
-    )
   attributes: dict[str, AttributeValue] = {
       GEN_AI_OPERATION_NAME: "invoke_workflow",
       GEN_AI_CONVERSATION_ID: conversation_id,
@@ -207,11 +203,14 @@ def _use_invoke_workflow_span(
   start_s = time.monotonic()
   workflow_span: Span | None = None
   try:
-    with tracer.start_as_current_span(
-        name=span_name,
-        attributes=attributes,
-        context=otel_context,
-    ) as span:
+    with (
+        tracer.start_as_current_span(
+            name=span_name,
+            attributes=attributes,
+            context=otel_context,
+        ) as span,
+        _mark_nested_workflows(),
+    ):
       workflow_span = span
       yield span
   finally:
@@ -221,3 +220,14 @@ def _use_invoke_workflow_span(
         nested=nested,
         error=sys.exc_info()[1],
     )
+
+
+@contextmanager
+def _mark_nested_workflows() -> Iterator[None]:
+  token = context_api.attach(
+      context_api.set_value(_ENTRYPOINT_WORKFLOW_KEY, True)
+  )
+  try:
+    yield
+  finally:
+    context_api.detach(token)