Allow UsageMetadata for the third-party language models via LangChain4j integration

[rhuan080]

Hello,

I'm using ADK 0.3.0 together with google-adk-langchain4j to work with multiple models. I noticed that response.usageMetadata() only works for Gemini. It would be very useful if usage metadata could be standardized and supported across the main models in the market.

[rhuan080]

PR 623