code: def test_gathering():
      config_list = autogen.config_list_from_json(
          OAI_CONFIG_LIST,
          file_location=KEY_LOC,
      )
      assistant1 = AssistantAgent(
          "assistant",
          system_message="You are a helpful assistant.",
          llm_config={
              "config_list": config_list,
              "model": "gpt-3.5-turbo-0613",
          },
      )
      assistant2 = AssistantAgent(
          "assistant",
          system_message="You are a helpful assistant.",
          llm_config={
              "config_list": config_list,
              "model": "gpt-3.5-turbo-0613",
          },
      )
      assistant3 = AssistantAgent(
          "assistant",
          system_message="You are a helpful assistant.",
          llm_config={
              "config_list": config_list,
              "model": "gpt-3.5-turbo-0613",
          },
      )
  
      assistant1.client.total_usage_summary = {
          "total_cost": 0.1,
          "gpt-35-turbo": {
              "cost": 0.1,
              "prompt_tokens": 100,
              "completion_tokens": 200,
              "total_tokens": 300,
          },
      }
      assistant2.client.total_usage_summary = {
          "total_cost": 0.2,
          "gpt-35-turbo": {
              "cost": 0.2,
              "prompt_tokens": 100,
              "completion_tokens": 200,
              "total_tokens": 300,
          },
      }
      assistant3.client.total_usage_summary = {
          "total_cost": 0.3,
          "gpt-4": {
              "cost": 0.3,
              "prompt_tokens": 100,
              "completion_tokens": 200,
              "total_tokens": 300,
          },
      }
  
      total_usage, _ = gather_usage_summary(
          [assistant1, assistant2, assistant3]
      )
  
      assert round(total_usage["total_cost"], 8) == 0.6
      assert round(total_usage["gpt-35-turbo"]["cost"], 8) == 0.3
      assert round(total_usage["gpt-4"]["cost"], 8) == 0.3
  
      # test when agent doesn't have client
      user_proxy = UserProxyAgent(
          name="ai_user",
          human_input_mode="NEVER",
          max_consecutive_auto_reply=2,
          code_execution_config=False,
          default_auto_reply="That's all. Thank you.",
      )
  
      total_usage, acutal_usage = gather_usage_summary([user_proxy])
  
file_path: autogen/test/agentchat/test_agent_usage.py
imports: from autogen.agent_utils import gather_usage_summary
  from autogen import AssistantAgent, UserProxyAgent
  from test_assistant_agent import KEY_LOC, OAI_CONFIG_LIST
  import pytest
  from conftest import skip_openai
  import autogen
  import io
  from contextlib import redirect_stdout
label_desc: Use image node_id 1 for calling a global function with example usage: test_gathering() without return types