Better custom model handling OpenAICallbackHandler (#4009)

Thanks @maykcaldas for flagging! think this should resolve #3988. Let me
know if you still see issues after next release.

tests/unit_tests/callbacks/test_openai_info.py

@@ -0,0 +1,46 @@
+import pytest
+
+from langchain.callbacks import OpenAICallbackHandler
+from langchain.llms.openai import BaseOpenAI
+from langchain.schema import LLMResult
+
+
+@pytest.fixture
+def handler() -> OpenAICallbackHandler:
+    return OpenAICallbackHandler()
+
+
+def test_on_llm_end(handler: OpenAICallbackHandler) -> None:
+    response = LLMResult(
+        generations=[],
+        llm_output={
+            "token_usage": {
+                "prompt_tokens": 2,
+                "completion_tokens": 1,
+                "total_tokens": 3,
+            },
+            "model_name": BaseOpenAI.__fields__["model_name"].default,
+        },
+    )
+    handler.on_llm_end(response)
+    assert handler.successful_requests == 1
+    assert handler.total_tokens == 3
+    assert handler.prompt_tokens == 2
+    assert handler.completion_tokens == 1
+    assert handler.total_cost > 0
+
+
+def test_on_llm_end_custom_model(handler: OpenAICallbackHandler) -> None:
+    response = LLMResult(
+        generations=[],
+        llm_output={
+            "token_usage": {
+                "prompt_tokens": 2,
+                "completion_tokens": 1,
+                "total_tokens": 3,
+            },
+            "model_name": "foo-bar",
+        },
+    )
+    handler.on_llm_end(response)
+    assert handler.total_cost == 0