Breaking change in 2.0.3 for override fields about mapped-types

[KuanWenChen]

OK, I think I found the problem.
_excludeMetadatas, _exposeMetadatas, _typeMetadatas should not be a Array
So, they should be overwriten by children.
I would create PR as soon as.

[vsamofal]

@KuanWenChen probably you can take a look, I understand that you were fixing something, but we also break some, so need to come up with some plan for all known cases to work

[KuanWenChen]

I thing problem is happen at value is not a iterable type. I am not sure why value is not a iterable type.
I thing this could fix it:

[existingRules, targetMetadataEntries].forEach((entries) => {
  for (const [valueKey, value] of entries) {
    const arrayValue = Array.isArray(value) ? value : [value];
    if (mergeMap.has(valueKey)) {
      mergeMap.get(valueKey)!.push(...arrayValue);
    } else {
      mergeMap.set(valueKey, arrayValue);
    }
  }
});

[kamilmysliwiec]

Could you create a PR for this issue?

[kamilmysliwiec]

Let's track this here #1225