Live Mode is missing usage_metadata and telemetry tracing

[emilmilow]

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)

[hangfei]

This is not supported yet. Will convert this to a feature request.

[emilmilow]

This is not supported yet. Will convert this to a feature request.

@hangfei check the PR, the data is there we just need to pass it to the LlmResponse. Also telemetry is a pretty easy fix, check the pr for that as well.

[ravi-incred]

@hangfei Waiting for this to get merged....

[hangfei]

token usage is supported here: 6e5c0eb.

@wukath to see if we need tracing and evaluate the PR.