add postfix metrics exporter

[konvergence]

New values metrics allow to export postfix metrics for prometheus
You can also generate a serviceMonitor if you use prometheus operator

• example of values for metrics

# when metrics is enabled, we mount subpath log from pvc into /var/log/mail
metrics:
  enabled: true
  image:
    name: blackflysolutions/postfix-exporter@sha256
    tag: 7ed7c0534112aff5b44757ae84a206bf659171631edfc325c3c1638d78e74f73
    pullPolicy: "IfNotPresent"

  resources:
    requests:
      memory: "256Mi"
    #  cpu: "100M"
    #limits:
    #  memory: "256Mi"
    #  cpu: "500M"


  serviceMonitor:
    enabled: true
    scrapeInterval: 15s

[funkypenguin]

Sorry about the delay, this looks like a great addition, thank you!