ExtractField extract last element of an array

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: Hello. I would like to visualize better user question from a LLM trace. I configured vercel ai telemetry https://ai-sdk.dev/docs/ai-sdk-core/telemetry it send as ai.prompt.messages similar to ``` ai.prompt.messages=[ {"role":"system", "content":"you are opencoder"], {"role":"user","content":[{"type":"text","text":"hello"}], {"role":"assistant","content":[{"type":"text","text":"hello"}], .... {"role":"user","content":[{"type":"text","text":"question"}], ``` **What would you like to be added**: In https://github.com/grafana/grafana/tree/1a1531ca5e92554d107bb1eb9df2f9fdc604d824/public/app/features/transformers/extractFields I would like to extract last user question. Something like ```[-1].content.text``` **Who is this feature for?** LLM analytics