How to get the 0-N number after pod-name in statusfulset deployment mode

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Hello,I have a question to ask you. When I deploy a zookeeper cluster, how can I directly obtain the 0-N value behind the pod-name through the environment variables in the yaml file, for example:
I deployed a zookeeper cluster in statefuleset mode. Their pod names are zookeeper-0, zookeeper-1 and zookeeper-2. How do I get the three numbers 0, 1 and 2 in each container?

The following is how to write environment variables in my yaml file

      env:
        - name: ZOO_SERVER_ID
          value: $(sub "$(hostname)" "zookeeper-")
          valueFrom:
            fieldRef:
              fieldPath: metadata.labels['statefulset.kubernetes.io/pod-name']

I found that I can only get the pod-name. Here is how I got it. Can anyone help me find out how to get the number?

Cluster information:

Kubernetes version: 1.18
Cloud being used: (put bare-metal if not on a public cloud) bare metal
Installation method: kubeadm
Host OS: centos7
docker and version: 19.03

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