docs: def provision(self, instance_id: str, service_details: ProvisionDetails, async_allowed: bool) -> ProvisionedServiceSpec:
          """Provision the new instance
          
          see openbrokerapi documentation
          
          Returns:
              ProvisionedServiceSpec
          """
          
          if service_details.plan_id == self._backend.config.UUID_PLANS_EXISTING_CLUSTER:
              # Provision the instance on an Existing Atlas Cluster
              
              # Find or create the instance
              instance = self._backend.find(instance_id)
              
              # Create the instance if needed
              return self._backend.create(instance, service_details.parameters, existing=True)
          
          # Plan not supported
          raise ErrPlanUnsupported(service_details.plan_id)