Custom Evaluations in LangChain: Using Phoenix for Observability

Hello! I am testing phoenix to observe a langchain chain with many steps. I have a question about evaluations. Is there a way to add custom evaluation to specific Runnables in my chain and show them in phoenix UI? I understand that span evaluations are kept separate from the spans and zipped to them at some point (using span ids?), but I could not find a way to get span ids generated along a langchain chain.