WHAT
Implement logic to expose and create relevant metrics for Postgres in GCP, in CRO.

HOW

• We should aim to follow a similar pattern as used in AWS see -  https://github.com/integr8ly/cloud-resource-operator/blob/master/pkg/providers/aws/provider_postgres.go#L317
• Part of this ticket would be investigation for similar metrics to expose from GCP that are currently listed from AWS. See the GCP reference for postgres metrics

TESTS
Unit tests added.

DONE
Relevant metrics are created and exposed matching the aws implementation.