Custom Cloud Function in Firebase

We used to have a very simple Function hosted on Firebase with a URL similar to: [login to view URL]{reqid} However this function seems to have disappeared and we have lost contact with our developer. The function simply queried a specific collection with a set of documents inside Firestore and returned the JSON structure through this custom Endpoint. We need to redo this API. The purpose is to recover chat conversations from a Web Backend (Admin Console). We need this API to grant access to a specific user or request header (whichever is simpler). We don´t care about server language scripting as long as function is built properly, it consumes the lowest possible resources and source code is provided. However we don´t want middleware, direct connection only. (We believe current functions are run on Node.js)