[exhentai] implement '"source": "metadata"' (#4902)

docs/configuration.rst

@@ -2557,6 +2557,8 @@ Description
     Selects an alternative source to download files from.
 
     * ``"hitomi"``:  Download the corresponding gallery from ``hitomi.la``
+    * ``"metadata"``:  Load only a gallery's metadata from the
+      `API <https://ehwiki.org/wiki/API#Gallery_Metadata>`_
 
 
 extractor.exhentai.tags

gallery_dl/extractor/exhentai.py

@@ -136,6 +136,8 @@ class ExhentaiGalleryExtractor(ExhentaiExtractor):
         source = self.config("source")
         if source == "hitomi":
             self.items = self._items_hitomi
+        elif source == "metadata":
+            self.items = self._items_metadata
 
         limits = self.config("limits", False)
         if limits and limits.__class__ is int:
@@ -223,6 +225,9 @@ class ExhentaiGalleryExtractor(ExhentaiExtractor):
         data["_extractor"] = HitomiGalleryExtractor
         yield Message.Queue, url, data
 
+    def _items_metadata(self):
+        yield Message.Directory, self.metadata_from_api()
+
     def get_metadata(self, page):
         """Extract gallery metadata"""
         data = self.metadata_from_page(page)