[BUG] Failure in TokenCollector.collect_openai_tokens_for_parent_span method

[vladfeigin]

Describe the bug
I'm using promptflow for trace collecting. When running the application it fails in collect_openai_tokens_for_parent_span method method with the error: "TypeError: unsupported operand type(s) for +: 'int' and 'dict'\n'}}], 'debugInfo': {'type': 'ScriptExecutionError', 'message': "Execution failure in 'rag_flow': (TypeError) unsupported operand type(s) for +: 'int' and 'dict'""

How To Reproduce the bug
from promptflow.tracing import start_trace

start_trace()
... code ...

Then from command line run: pf flow serve --source ./ --port 8080 --host localhost
at some point the failure happens

Running Information(please complete the following information):

{
"promptflow": "1.16.2",
"promptflow-azure": "1.16.1",
"promptflow-core": "1.16.2",
"promptflow-devkit": "1.16.2",
"promptflow-tracing": "1.16.2"
}

OS:
ProductName: macOS
ProductVersion: 15.2
BuildVersion: 24C101

Python 3.11.11

Additional context
Issue explanation: The root cause is in this code :
merged_tokens = {
# When token count is None for some reason, we should default to 0.
key: (self._span_id_to_tokens[parent_span_id].get(key, 0) or 0) + (tokens.get(key, 0) or 0)
for key in set(self._span_id_to_tokens[parent_span_id]) | set(tokens)
}

this is because can be that "tokens" dictionary is not flat dictionary but could contain another dictionaries.
For example: {'completion_tokens': 112, 'prompt_tokens': 587, 'total_tokens': 699, 'completion_tokens_details': {'audio_tokens': 0, 'reasoning_tokens': 0, 'accepted_prediction_tokens': 0, 'rejected_prediction_tokens': 0}, 'prompt_tokens_details': {'audio_tokens': 0, 'cached_tokens': 0}} . In this case tokens dictionary contains "completion_tokens_details and prompt_tokens_details which are dictionaries. However the code attempting 0 + dictionary, and this crash the code.