code: public static List<ExtensionElementProvider<ExtensionElement>> getExtensionProviders() {
          List<ExtensionElementProvider<ExtensionElement>> providers = new ArrayList<>(extensionProviders.size());
          providers.addAll(extensionProviders.values());
          return providers;
      }
docs: Returns an unmodifiable collection of all PacketExtensionProvider instances. Each object
  in the collection will either be a PacketExtensionProvider instance, or a Class object
  that implements the PacketExtensionProvider interface.
  
  @return all PacketExtensionProvider instances.