Add user agent to prompty standalone execution (#3475)

# Description

Add capability to pass extra headers when using prompty

# 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-core/promptflow/core/_prompty_utils.py

@@ -146,7 +146,7 @@ def convert_prompt_template(template, inputs, api):
 def prepare_open_ai_request_params(model_config, template, connection):
     params = copy.copy(model_config.parameters)
     if isinstance(connection, AzureOpenAIConnection):
-        params["extra_headers"] = {"ms-azure-ai-promptflow-called-from": "promptflow-core"}
+        params.setdefault("extra_headers", {}).update({"ms-azure-ai-promptflow-called-from": "promptflow-core"})
     params["model"] = model_config._model
 
     if model_config.api == "completion":

src/promptflow-evals/promptflow/evals/evaluators/_relevance/_relevance.py

@@ -9,6 +9,10 @@
 
 from promptflow.client import load_flow
 from promptflow.core import AzureOpenAIModelConfiguration
+try:
+    from ..._user_agent import USER_AGENT
+except ImportError:
+    USER_AGENT = None
 
 
 class RelevanceEvaluator:
@@ -44,7 +48,13 @@ def __init__(self, model_config: AzureOpenAIModelConfiguration):
         if model_config.api_version is None:
             model_config.api_version = "2024-02-15-preview"
 
-        prompty_model_config = {"configuration": model_config}
+        prompty_model_config = {
+            "configuration": model_config,
+        }
+
+        prompty_model_config.update({"parameters": {"extra_headers": {"x-ms-user-agent": USER_AGENT}}})\
+            if USER_AGENT and isinstance(model_config, AzureOpenAIModelConfiguration) else None
+
         current_dir = os.path.dirname(__file__)
         prompty_path = os.path.join(current_dir, "relevance.prompty")
         self._flow = load_flow(source=prompty_path, model=prompty_model_config)