With the exception of DefaultMetadataDetails.Key, metadata about a ValueType and its Nullable<T> pair is identical and should not be stored separately. Further the ModelMetadataIdentity of a Nullable<T> property and it's ModelExplorer should not change when HasValue changes.

This is a follow-on from #2539. That issue relates to when ViewDataDictionary requests a new ModelMetadata instance. This is about making the returned metadata (when required) as close as possible for bool and bool?. In addition IMetadataDetailsProvider instances should be queried once about T and not twice i.e. not also about Nullable<T>.

There may be a slight performance improvement in not querying the IMetadataDetailsProvider instances twice. But the main issue is keeping the metadata consistent.