Gemini Model not returning Usage Metadata in Streaming responses

[chaskyhoffmann]

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Describe the bug
When using streaming mode, the Gemini Model does not return the usageMetadata in the LlmResponse. When stream=true the usageMetadata should return in the final aggregated response.

To Reproduce
Steps to reproduce the behavior:

1. Create an LlmAgent with the Gemini model.
2. In the RunConfig set the streaming model to RunConfig.StreamingMode.SSE
3. observe the LlmResponse in afterModelCallback of a plugin or agent.
4. See that both for partial and final LLM responses, usageMetadata is Optional.Empty

Expected behavior
The final and non-partial LlmResponse should include the usageMetadata.

Desktop (please complete the following information):

• OS: MacOS
• Java version:
• ADK version(see maven dependency): 0.5.0

Additional context
The python version of ADK returns the usageMetadata correctly.
The usageMetadata is returned from the Gemini API in the last chunk.