Add token usage to gemini (streaming), litellm and anthropic

Also included a token_usage sample that showcases the token usage of subagents with different models under a parent agent.

PiperOrigin-RevId: 759347015

src/google/adk/flows/llm_flows/base_llm_flow.py

@@ -218,7 +218,7 @@ class BaseLlmFlow(ABC):
 
       When the model returns transcription, the author is "user". Otherwise, the
       author is the agent name(not 'model').
-      
+
       Args:
         llm_response: The LLM response from the LLM call.
       """