Firestore-Cloud Functions get a top layer file name

Question

Have a nice day. First; I would like to thank this user. https://stackoverflow.com/a/52982781/11320646 I want you to know that I'm new to writing code. I have to train a project. The function of these codes(include link, "solution" tittle) is this:

customers/{id} counts the consumers here and write count here: metadatas/customers, {count:123456}.

But i want to add counter here:

groups/{groupID}/posts/{postID}

and ı should write count here:

metadatas/{groupID}/counts, {count:123456}

But these codes write:

metadatas/posts, {count:123456} 

How to add the groupID layer?

I've tried many things, but I haven't succeeded.

I have successfully done counting with these codes, I have activated the retry option if it fails. I got a great result. At the same time more than 100 post was added and the result is excellent. my codes are here:

const executeOnce = (change, context, task) => {
    const eventRef = firestore.collection('events').doc(context.eventId);

    return firestore.runTransaction(t =>
        t
         .get(eventRef)
         .then(docSnap => (docSnap.exists ? null : task(t)))
         .then(() => t.set(eventRef, { processed: true }))
    );
};

const documentCounter = collectionName => (change, context) =>
    executeOnce(change, context, t => {
        // on create
        if (!change.before.exists && change.after.exists) {
            return t
                    .get(firestore.collection('counts')
                    .doc(collectionName))
                    .then(docSnap =>
                        t.set(docSnap.ref, {
                            count: ((docSnap.data() && docSnap.data().count) || 0) + 1
                        }));
        // on delete
        } else if (change.before.exists && !change.after.exists) {
            return t
                     .get(firestore.collection('counts')
                     .doc(collectionName))
                     .then(docSnap =>
                        t.set(docSnap.ref, {
                            count: docSnap.data().count - 1
                        }));
        }

        return null;
    });
exports.customersCounter = functions.firestore
    .document('denemeicerikler/{grupID}/icerikler/{postID}')
    .onWrite(documentCounter('icerikler'));