[prometheus-postgres-exporter] introduce podmonitoring

[joawin]

What this PR does / why we need it

• Allows use of PodMonitor instead of the traditional ServiceMonitor.

Special notes for your reviewer

Checklist

• DCO signed
• Chart Version bumped
• Title of the PR starts with chart name (e.g. [prometheus-couchdb-exporter])

[zeritti]

Thank you, @joawin, for your PR. I am of the opinion that we should not include custom resources from outside of monitoring.coreos.com in the chart as is the case of PodMonitoring. Please, consider using the field extraManifests for any additional resources you'd like to deploy.

[zeritti]

@joawin By using the values' field extraManifests I meant using this field for your custom resources in your own values at a release install/upgrade. In other words, there is no need to introduce PodMonitoring in the chart in any form.

[joawin]

@zeritti why not. We want to deploy it on GCP environment where we need it as Servicemonitor is not supported.
For Blackbox-Exporter it is also implemented