unsubscribe function for subscriptionField

I want to implement my own subscription manager. I can't use the plugin because I need a different API. How do I know if a subscription was ended / lost?

I need something like this:

builder.subscriptionField('station', (t) =>
  t.field({
    args: {
      where: t.arg({
        type: "StationWhereInput"
      })
    },
    type: 'StationSubscriptionPayload',
    subscribe: (_parent: any, args: any, context: any, { fieldName }: ResolveShape) => {
      const channel = uid()
      SUBSCRIPTIONS[channel] = { ...args, channelName: fieldName }
      return pubsub.subscribe(channel)
    },
    unsubscribe: ???,
    resolve: async ({ node, mutation }) => {
      return {
        mutation,
        node
      }
    }
  })
)

I need to track active Subscriptions and want to know when someone stopped a subscription. (remove it from my SUBSCRIPTIONS object).

Or if this is not possible, is there something like apollo's withFilter? That would be even cooler. https://www.apollographql.com/docs/apollo-server/data/subscriptions/#filtering-events