Prometheus export translation strategy property should be subschema in $defs

> Unrelated: enum type should be broken out as a dedicated type. 
> 
> https://github.com/open-telemetry/opentelemetry-configuration/blob/main/CONTRIBUTING.md#schemas-and-subschemas 

 _Originally posted by @jack-berg in [#376](https://github.com/open-telemetry/opentelemetry-configuration/pull/376/files#r2520912606)_