how to set environment tag for kubernetes deployment in datadog

I need to set env tags based on service like ist, preprod, prod etc for kubenernetes deployment.yaml files.

value: {{ .Values.env }} 

How and where to set it.

This is my whole deployment.yaml file - 
apiVersion: apps/v1
kind: Deployment
metadata:
  name: {{ template "caas-arm-2.fullname" . }}
  labels:
    app: {{ template "caas-arm-2.name" . }}
    chart: {{ template "caas-arm-2.chart" . }}
    release: {{ .Release.Name }}
    heritage: {{ .Release.Service }}
spec:
  replicas: {{ .Values.replicaCount }}
  selector:
    matchLabels:
      app: {{ template "caas-arm-2.name" . }}
      release: {{ .Release.Name }}
  template:
    metadata:
      annotations:
        vault.security.banzaicloud.io/vault-skip-verify: "true"
        vault.security.banzaicloud.io/vault-role: vault-read
        vault.security.banzaicloud.io/vault-path: "{{ .Values.env}}-ux"
        vault.security.banzaicloud.io/vault-env-from-path: "cluster-secrets/data/{{.Values.env}}/{{ template "caas-arm-2.fullname" . }}"
        vault.security.banzaicloud.io/vault-addr: https://vault.gatethree.io:443
        alpha.vault.security.banzaicloud.io/secret-reload-count: "1"
        alpha.vault.security.banzaicloud.io/reload-on-secret-change: "true"
      labels:
        app: {{ template "caas-arm-2.name" . }}
        release: {{ .Release.Name }}
    spec:
      serviceAccountName: vault-read
      topologySpreadConstraints:
        - maxSkew: 1
          topologyKey: kubernetes.io/hostname
          whenUnsatisfiable: DoNotSchedule
          labelSelector:
            matchLabels:
              app: {{ template "caas-arm-2.name" . }}     
      containers:
        - name: {{ .Chart.Name }}
          image: "{{ .Values.image.repository }}:{{ .Values.image.tag }}"
          imagePullPolicy: {{ .Values.image.pullPolicy }}
          resources:
            limits:
              memory: {{ .Values.memory.limit }}
          ports:
            - name: http
              containerPort: {{ .Values.service.targetPort }}
              protocol: TCP
          livenessProbe:
            initialDelaySeconds: 40
            periodSeconds: 20
            httpGet:
              path: /healthz
              port: {{ .Values.service.targetPort }}
          readinessProbe:
            initialDelaySeconds: 40
            periodSeconds: 20
            httpGet:
              path: /healthz
              port: {{ .Values.service.targetPort }}
          env:
            - name: DD_AGENT_HOST
              valueFrom:
                fieldRef:
                  fieldPath: status.hostIP
            {{- $root := . }}
            {{- range $ref, $values := .Values.secrets }}
            {{- range $key, $value := $values }}
            - name: {{ $ref | upper }}_{{ $key | upper }}
              valueFrom:
                secretKeyRef:
                  name: {{ template "caas-arm-2.fullname" $root }}-{{ $ref }}
                  key: {{ $key }}
            {{- end }}
            {{- end }}
    {{- with .Values.imagePullSecrets }}
      imagePullSecrets:
{{ toYaml . | indent 8 }}
    {{- end }}
    {{- with .Values.nodeSelector }}
      nodeSelector:
{{ toYaml . | indent 8 }}
    {{- end }}
    {{- with .Values.affinity }}
      affinity:
{{ toYaml . | indent 8 }}
    {{- end }}
    {{- with .Values.tolerations }}
      tolerations:
{{ toYaml . | indent 8 }}
    {{- end }}