Update the `pg_replication` metric

[gbartolini]

Once the patch for https://github.com/cloudnative-pg/cloudnative-pg/issues/1814 is merged in cloudnativepg, update the values.xml file.

(We should think about a way to automatically update the queries content in the values.xml file from the value in cloudnativepg's docs/src/samples/cluster-example-monitoring.yaml file