Fix telemetry (#787)

# What does this PR do?

PR fixes couple of issues with telemetry:
1) The REST refactor changed the method from get_span_tree to
query_span_tree, which is causing the server side to return empty spans
2) Library client has introduced a new event loop, which required
changing the location of where start and end trace are called


## Test Plan

LLAMA_STACK_CONFIG="/Users/dineshyv/.llama/distributions/llamastack-fireworks/fireworks-run.yaml"
pytest -v tests/client-sdk/agents/test_agents.py -k
"test_builtin_tool_web_search"


And querying for spans from the agent run using the library client.

llama_stack/distribution/library_client.py

@@ -269,7 +269,7 @@ class AsyncLlamaStackAsLibraryClient(AsyncLlamaStackClient):
             set_request_provider_data(
                 {"X-LlamaStack-Provider-Data": json.dumps(self.provider_data)}
             )
-        await start_trace(options.url, {"__location__": "library_client"})
+
         if stream:
             response = await self._call_streaming(
                 cast_to=cast_to,
@@ -281,7 +281,6 @@ class AsyncLlamaStackAsLibraryClient(AsyncLlamaStackClient):
                 cast_to=cast_to,
                 options=options,
             )
-        await end_trace()
         return response
 
     def _find_matching_endpoint(self, method: str, path: str) -> tuple[Any, dict]:
@@ -323,7 +322,11 @@ class AsyncLlamaStackAsLibraryClient(AsyncLlamaStackClient):
         matched_func, path_params = self._find_matching_endpoint(options.method, path)
         body |= path_params
         body = self._convert_body(path, options.method, body)
-        result = await matched_func(**body)
+        await start_trace(options.url, {"__location__": "library_client"})
+        try:
+            result = await matched_func(**body)
+        finally:
+            await end_trace()
 
         json_content = json.dumps(convert_pydantic_to_json_value(result))
         mock_response = httpx.Response(
@@ -366,10 +369,14 @@ class AsyncLlamaStackAsLibraryClient(AsyncLlamaStackClient):
         body = self._convert_body(path, options.method, body)
 
         async def gen():
-            async for chunk in await func(**body):
-                data = json.dumps(convert_pydantic_to_json_value(chunk))
-                sse_event = f"data: {data}\n\n"
-                yield sse_event.encode("utf-8")
+            await start_trace(options.url, {"__location__": "library_client"})
+            try:
+                async for chunk in await func(**body):
+                    data = json.dumps(convert_pydantic_to_json_value(chunk))
+                    sse_event = f"data: {data}\n\n"
+                    yield sse_event.encode("utf-8")
+            finally:
+                await end_trace()
 
         mock_response = httpx.Response(
             status_code=httpx.codes.OK,