fix: treat usage_metadata.total_token_count of vertex ai as total (#1208)

fix(langchain): treat `usage_metadata.total token count` of vertex ai as total

Author: gion-pop

langfuse/langchain/CallbackHandler.py

@@ -793,6 +793,7 @@ def _parse_usage_model(usage: typing.Union[pydantic.BaseModel, dict]):
         # https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/get-token-count
         ("prompt_token_count", "input"),
         ("candidates_token_count", "output"),
+        ("total_token_count", "total"),
         # Bedrock: https://docs.aws.amazon.com/bedrock/latest/userguide/monitoring-cw.html#runtime-cloudwatch-metrics
         ("inputTokenCount", "input"),
         ("outputTokenCount", "output"),