Add GKE monitoring config to the helm chart

[liu-cong]

Step5 in the GKE user guide configured necessary resources for enabling EPP metrics on GCP dashboards. Ideally these can be added to the helm chart. See #1425 for an attempt to add monitoring configs.

I am expecting we have an overall monitoring config with both common and provider specific configs, something like:

inferenceExtension:
  monitoring:
    path: "metrics"
    ... # other common params
    prometheus:
       # prometheus specific params
    gke: 
      # gke specific config, if any

[liu-cong]

cc @JeffLuoo @rahulgurnani

[sallyom]

I've added the option of monitoring.gke in the #1425 PTAL - I don't have a GKE env to test on, however.