Fix bug in token usage merging logic for promptflow-tracing SDK (#3793)

# Description

The token calculation logic in promptflow-tracing merges two
dictionaries related to token usage.
i.e. Given these two dictionaries:
```json
{ "completion_tokens": 146, "prompt_tokens": 166, "total_tokens": 312 }
```
```json
{ "completion_tokens": 10, "prompt_tokens": 10, "total_tokens": 20 }
```
The merged result will be:
```json
{ "completion_tokens": 156, "prompt_tokens": 176, "total_tokens": 332 }
```

However, for some customized models, there might be more additional
fields populated in the dictionaries, like:
```json
{ "completion_tokens": 146, "prompt_tokens": 166, "total_tokens": 312, "completion_tokens_details": null }
```
Which causes the merging logic crash:


![image](https://github.com/user-attachments/assets/c1777ebf-db00-4c3c-a449-7a72274ffc0c)

This PR is to fix the issue. It converts `null`s to `0`s to avoid Python
from raising errors.

# All Promptflow Contribution checklist:
- [x] **The pull request does not introduce [breaking changes].**
- [ ] **CHANGELOG is updated for new features, bug fixes or other
significant changes.**
- [x] **I have read the [contribution
guidelines](https://github.com/microsoft/promptflow/blob/main/CONTRIBUTING.md).**
- [x] **I confirm that all new dependencies are compatible with the MIT
license.**
- [ ] **Create an issue and link to the pull request to get dedicated
review from promptflow team. Learn more: [suggested
workflow](../CONTRIBUTING.md#suggested-workflow).**

## General Guidelines and Best Practices
- [ ] Title of the pull request is clear and informative.
- [ ] There are a small number of commits, each of which have an
informative message. This means that previously merged commits do not
appear in the history of the PR. For more information on cleaning up the
commits in your PR, [see this
page](https://github.com/Azure/azure-powershell/blob/master/documentation/development-docs/cleaning-up-commits.md).

### Testing Guidelines
- [ ] Pull request includes test coverage for the included changes.

---------

Co-authored-by: Hanchi Wang <[email protected]>
Co-authored-by: Hanchi Wang <[email protected]>

src/promptflow-tracing/CHANGELOG.md

@@ -1,5 +1,9 @@
 # promptflow-tracing package
 
+## v1.16.1 (2024.10.8)
+
+- Fix token count issue when the value is None.
+
 ## v1.10.0 (2024.04.26)
 
 ### Features Added

src/promptflow-tracing/promptflow/tracing/_trace.py

@@ -141,7 +141,8 @@ def collect_openai_tokens_for_parent_span(self, span):
             with self._lock:
                 if parent_span_id in self._span_id_to_tokens:
                     merged_tokens = {
-                        key: self._span_id_to_tokens[parent_span_id].get(key, 0) + tokens.get(key, 0)
+                        # 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)
                     }
                     self._span_id_to_tokens[parent_span_id] = merged_tokens