[charts/metabase] : Fixing servicemonitor according to prometheus and k8s spec

[William-LP]

Hello !

I'm trying to install the metabase chart but I run into this error when enabling the serviceMonitor as below :

metabase:
  monitoring:
    enabled: true
    serviceMonitor:
      enabled: true
    port: 9191

Error: UPGRADE FAILED: error validating "": error validating data: [ValidationError(ServiceMonitor.spec.selector): unknown field "app" in com.coreos.monitoring.v1.ServiceMonitor.spec.selector, ValidationError(ServiceMonitor.spec.selector): unknown field "release" in com.coreos.monitoring.v1.ServiceMonitor.spec.selector]

As per official kubernetes and prometheus spec, the matchLabels field is missing. This PR provides a fix.

I've tested the code and it works like a charm ! :)

[pmint93]

Thanks a lot !