docs: public Set<MetadataProvider> getMetadataProviders(MediaDetails sourceMedia) {
          String key = (sourceMedia == null)? "" : sourceMedia.hashKey();
          Set<MetadataProvider> result = metadataProviders.get(key);
          if (result == null) {
              return Collections.emptySet();
          }
          return Collections.unmodifiableSet(new HashSet<MetadataProvider>(result));
      }