Observability throws an error setting metrics when metrics plan

[JorTurFer]

Description

Based on service plan docs, the retention of the metrics should be configurable for Monitoring XXXX plans but the current terraform resource raises and error.

Steps to reproduce

Try to plan an observability resource with service plan "Observability-Monitoring-Basic-EU01" just setting any retention time for metrics and execute terraform plan.

Actual behavior

An error is shown saying that the option isn't supported for the plan

Expected behavior

No errors

## Additional info

The problem is that this validation isn't correct

terraform-provider-stackit/stackit/internal/services/observability/instance/resource.go

Lines 927 to 929 in da2bb41

	if metricsRetentionDays != nil || metricsRetentionDays5mDownsampling != nil || metricsRetentionDays1hDownsampling != nil {
	core.LogAndAddError(ctx, &resp.Diagnostics, "Error validating plan", fmt.Sprintf("Plan (%s) does not support configuring metrics retention days. Remove this from your config or use a different plan.", *plan.Name))
	}

It verifies if the storage for logs and traces is 0 and then, it assumes that configuring metrics isn't possible either, when in theory is possible

[Fyusel]

Hi @JorTurFer,
thank you for reporting this issue.

I guess that this is fixed with: #1148
If that is not the case please feel free to open this issue again.