fix(telemetry): query_spans (#1831)

# What does this PR do?
https://github.com/meta-llama/llama-stack/pull/1828 removed
__root_span__ attribute which is still needed

## Test Plan
added telemetry integration test


LLAMA_STACK_CONFIG=http://localhost:5001 pytest -s -v
tests/integration/telemetry --safety-shield meta-llama/Llama-Guard-3-8B
--text-model accounts/fireworks/models/llama-v3p3-70b-instruct

llama_stack/providers/inline/telemetry/meta_reference/telemetry.py

@@ -202,6 +202,8 @@ class TelemetryAdapter(TelemetryDatasetMixin, Telemetry):
                     parent_span_id = int(event.payload.parent_span_id, 16)
                     parent_span = _GLOBAL_STORAGE["active_spans"].get(parent_span_id)
                     context = trace.set_span_in_context(parent_span)
+                else:
+                    event.attributes["__root_span__"] = "true"
 
                 span = tracer.start_span(
                     name=event.payload.name,