I have an ingress resource like the following:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: "app-ingress"
  labels:
    app: "app"
  annotations:
    kubernetes.io/ingress.class: nginx
    nginx.ingress.kubernetes.io/backend-protocol: "GRPC"
    cert-manager.io/cluster-issuer: letsencrypt
spec:
  tls:
    - hosts:
        - api.example.com
      secretName: tls-secret
  rules:
    - host: api.example.com
      http:
        paths:
          - path: /grpc
            pathType: ImplementationSpecific
            backend:
              service:
                name: "app-server"
                port:
                  number: 50051
          - path: /callback
            pathType: ImplementationSpecific
            backend:
              service:
                name: "app-server"
                port:
                  number: 80

app-server runs two services (on the same pod), a grpc web server and a http web server. To start, a client makes a request to api.example.com/grpc. To fulfill the request, the server makes a request to a third party service, waits for a callback on api.example.com/callback, and then completes the client's request (api.example.com/callback is only accessed by the third party service, not the client).

The question is, how can I specifically route api.example.com/callback back to the same pod that made the initial request? Obviously if pod1 were to make a request to the third party service and pod2 were to receive the callback, pod1 would never receive the callback and the client request would have to timeout. I can attach information to the callback I receive from the third party service, so is there a way I could attach some sort of pod id so that kubernetes will route the callback back to the pod that initiated it?

I've looked at maybe using StatefulSets, but I'm not sure exactly how this would work. Or would it be better to have sort of "notification pod" that receives callbacks and routes them to the correct pod?