LangChain Middleware No-Op Nodes

[MaxStenklyft]

Recently upgraded to langchain 1.2 and noticed that openllmetry was creating no-op nodes in my graph for every middleware regardless of if the middleware implemented a given function. For example, one middleware may only be applicable for before_agent but openllmetry was creating nodes for all all permutations of that middleware (before_agent, after_agent, before_model, after_model).

Extra nodes negatively impact the graph visibility, latency, and recursion limits. I've implemented a fix below and would love some feedback.

#4000