Describe the bug
When using ADK with Gemini Live API, the usage_metadata field is always None in Event objects, making it impossible to track token usage for cost monitoring and observability. Additionally, telemetry tracing doesn't work properly for live events, with token usage data being lost instead of creating individual spans for each event.

To Reproduce
Steps to reproduce the behavior:

1. Install google-adk
2. Create a voice agent using gemini-live-2.5-flash model:

   from google.adk.agents import Agent
   agent = Agent(
       name="voice_assistant_agent",
       model="gemini-live-2.5-flash", # or any other live model
       tools=[some_tool]
   )

3. Run the agent and process live events:

   async for event in live_events:
       print(f"Usage metadata: {event.usage_metadata}")  # Always None

4. CheckOpenTelemetry traces for token usage
5. Observe that token usage is not tracked in telemetry spans

Expected behavior

• event.usage_metadata should contain actual token usage data from the Gemini Live API response
• Each live event should create a separate OpenTelemetry span with token usage metrics (gen_ai.usage.input_tokens, gen_ai.usage.output_tokens, gen_ai.usage.total_tokens)