fix: ensure usage is requested if telemetry is enabled (#3571)

# What does this PR do?
Refs: https://github.com/llamastack/llama-stack/issues/3420

When telemetry is enabled the router uncondionally expects the usage
attribute to be availble and fails if it is not present.

Usage is not currently being requested by litellm_openai_mixin.py for
streaming requests when using the responses API which means that
providers like vertexai fail if telemetry is enabled and streaming is
used.

This is part of the required fix. Other part is in liteLLM, will plan to
submit PR for that soon.

## Test Plan
I applied this change along with the change for litellm in a llama stack
deployment and validated that I could make streaming requests through
the responses API to a gemini model and they would succeed instead of
failing due to the missing usage attribute when telemetry is enabled.

Signed-off-by: Michael Dawson <midawson@redhat.com>

llama_stack/providers/utils/inference/litellm_openai_mixin.py

@@ -374,6 +374,14 @@ class LiteLLMOpenAIMixin(
         top_p: float | None = None,
         user: str | None = None,
     ) -> OpenAIChatCompletion | AsyncIterator[OpenAIChatCompletionChunk]:
+        # Add usage tracking for streaming when telemetry is active
+        from llama_stack.providers.utils.telemetry.tracing import get_current_span
+
+        if stream and get_current_span() is not None:
+            if stream_options is None:
+                stream_options = {"include_usage": True}
+            elif "include_usage" not in stream_options:
+                stream_options = {**stream_options, "include_usage": True}
         model_obj = await self.model_store.get_model(model)
         params = await prepare_openai_completion_params(
             model=self.get_litellm_model_name(model_obj.provider_resource_id),