Fix spans_metric resource include_percentiles property

[VenelinMartinov]

The property was being set by the provider when the user doesn't set it but is not marked as Computed in the schema. This results in an error on apply:

╷
│ Error: Provider produced inconsistent result after apply
│ 
│ When applying changes to datadog_spans_metric.my_backup_success, provider
│ "provider[\"registry.terraform.io/datadog/datadog\"]" produced an unexpected new value:
│ .compute.include_percentiles: was null, but now cty.False.
│ 
│ This is a bug in the provider, which should be reported in the provider's own issue tracker.
╵
Error: exit status 1

This change fixes that by setting the property as Computed in the schema. Let me know if you have any feedback here!

fixes #2706