[alertmanager] Ability to pass existing persistent volume claim for a single replica statefulset

[NandigamAbhishek]

Is your feature request related to a problem ?

It would be great to use an existing persistent volume (if there is) in statefulset yaml file. I know this would mean that the statefulset should have a single pod running.

Describe the solution you'd like.

1. Add a persistentVolumeClaim block below the claimTemplate in statefulset.yaml

{{- if and ( .Values.persistence.enabled) (eq .Values.persistence.existingClaim "") }}
volumeClaimTemplates:
- metadata:
name: storage
spec:
accessModes:
{{- toYaml .Values.persistence.accessModes | nindent 10 }}
resources:
requests:
storage: {{ .Values.persistence.size }}
{{- if .Values.persistence.storageClass }}
{{- if (eq "-" .Values.persistence.storageClass) }}
storageClassName: ""
{{- else }}
storageClassName: {{ .Values.persistence.storageClass }}
{{- end }}
{{- end }}
{{- else if and (.Values.persistence.enabled) (not (eq .Values.persistence.existingClaim "")) (eq .Values.replicaCount 1) }}
- name: storage
persistentVolumeClaim:
claimName: {{ .Values.persistence.existingClaim }}
{{- else }}
- name: storage
emptyDir: {}
{{- end }}

2. Add a existingClaim key under persistence inside values.yaml

persistence:
enabled: true
accessModes:
- ReadWriteOnce
size: 50Mi
existingClaim: ""

Describe alternatives you've considered.

Maybe the logic of applying the condition can be better. Not sure.

Additional context.

This is a suggestion. I have tested using existing claim with azurefile-csi storage class in an aks cluster with single alertmanager pod. It works as expected.