Fix deprecated token callback, add deprecation on cost feature

Author: Yun-Kim

Diff for: ddtrace/contrib/internal/langchain/patch.py

@@ -129,11 +129,18 @@ def _tag_openai_token_usage(
     total_cost = span.get_metric(TOTAL_COST) or 0
     if not propagate and get_openai_token_cost_for_model:
         try:
-            completion_cost = get_openai_token_cost_for_model(
-                span.get_tag(MODEL),
-                span.get_metric(COMPLETION_TOKENS),
-                is_completion=True,
-            )
+            if parse_version(langchain_community.__version__) >= (0, 3, 13):
+                completion_cost = get_openai_token_cost_for_model(
+                    span.get_tag(MODEL),
+                    span.get_metric(COMPLETION_TOKENS),
+                    token_type=2,
+                )
+            else:
+                completion_cost = get_openai_token_cost_for_model(
+                    span.get_tag(MODEL),
+                    span.get_metric(COMPLETION_TOKENS),
+                    is_completion=True,
+                )
             prompt_cost = get_openai_token_cost_for_model(span.get_tag(MODEL), span.get_metric(PROMPT_TOKENS))
             total_cost = completion_cost + prompt_cost
         except ValueError:

Diff for: releasenotes/notes/deprecate-langchain-openai-cost-042f3a04cda9d23b.yaml

@@ -0,0 +1,5 @@
+---
+deprecations:
+  - |
+    langchain: The `langchain.tokens.total_cost` span metric for OpenAI calls is now deprecated and will be removed in `ddtrace==4.0`. 
+    For cost estimation of OpenAI calls, enable LLM Observability.