Ability to collect defined metadata property names

Is there a way to collect the decorated property names from a prototype without knowing the property names in advance?

It is a useful feature for an orm, ioc and other stuff.

Right know Reflect.getMetadataKeys(fn.prototype) returns an empty array. It would be useful to return the annotated property names if the second argument is undefined.
Maybe Reflect.getMetadataPropertyNames(fn.prototype)

Example:

 const fn:Function = ...;

 const propertyNames = Reflect.getMetadataKeys(fn.prototype);
 for (const name of propertyNames){
     const cachable = Reflect.hasMetadata("CachableMetaKey", fn.prototype, name);
     if (cachable) {
       //do stuff
     }
 }

Start by reading the existing Reflect.getMetadataKeys(fn.prototype) behavior and the related metadata reflection API semantics. Compare the proposed optional-second-argument behavior with the alternative Reflect.getMetadataPropertyNames(fn.prototype) entry point, then define which API should expose decorated property names and how completion would be verified.