[pf-evals] Fix the user agent not populated (#3309)

# Description

Please add an informative description that covers that changes made by
the pull request and link all relevant issues.

# All Promptflow Contribution checklist:
- [ ] **The pull request does not introduce [breaking changes].**
- [ ] **CHANGELOG is updated for new features, bug fixes or other
significant changes.**
- [ ] **I have read the [contribution guidelines](../CONTRIBUTING.md).**
- [ ] **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.

src/promptflow-evals/promptflow/evals/evaluate/_evaluate.py

@@ -372,14 +372,14 @@ def evaluate(
         _validate_columns(input_data_df, evaluators, target=None, evaluator_config=evaluator_config)
 
     # Batch Run
-    evaluator_info = {}
+    evaluators_info = {}
     use_thread_pool = kwargs.get("_use_thread_pool", True)
     batch_run_client = CodeClient() if use_thread_pool else pf_client
 
     with BatchRunContext(batch_run_client):
         for evaluator_name, evaluator in evaluators.items():
-            evaluator_info[evaluator_name] = {}
-            evaluator_info[evaluator_name]["run"] = batch_run_client.run(
+            evaluators_info[evaluator_name] = {}
+            evaluators_info[evaluator_name]["run"] = batch_run_client.run(
                 flow=evaluator,
                 run=target_run,
                 evaluator_name=evaluator_name,
@@ -388,10 +388,14 @@ def evaluate(
                 stream=True,
             )
 
+        # get_details needs to be called within BatchRunContext scope in order to have user agent populated
+        for evaluator_name, evaluator_info in evaluators_info.items():
+            evaluator_info["result"] = batch_run_client.get_details(evaluator_info["run"], all_results=True)
+
     # Concatenate all results
     evaluators_result_df = None
-    for evaluator_name, evaluator_info in evaluator_info.items():
-        evaluator_result_df = batch_run_client.get_details(evaluator_info["run"], all_results=True)
+    for evaluator_name, evaluator_info in evaluators_info.items():
+        evaluator_result_df = evaluator_info["result"]
 
         # drop input columns
         evaluator_result_df = evaluator_result_df.drop(